According to this article from Dolphin Music, "Another fuzz pedal the Beatles have used was an WEM Pep Rush, reportedly used on the Paperback Writer recordings. There is a photograph, taken around that same era, shows Lennon fiddling with the Pep Rush fuzz pedal." Another photograph can be found here.

Mentioned in the official product description of the Rush Amps reissue and this June 17, 2021 Reverb.com article written "IN PARTNERSHIP WITH RUSHAMPS." Pepe Rush's own memory of the delivery is given in David Toop's December 2007 essay "Rush Pep Box".

Leonardo Music Journal, Vol. 17, pgs. 21-23, "Rush Pep Box"

The original model, manufactured by Rush himself, seems to have been used by a small number of bands, hence its obscurity. The Beatles had two, psychedelic pop act Pinkerton’s Assorted Colours are said to have used one and Rush remembers delivering two to Cecil Sharp House, the English Folk Song and Dance Society, where The Shadows were rehearsing, although no trace of its barely controllable, harsh distortion is discernible on any of their recordings.

Rush Amps Rush Pepbox Product Description

The RUSH PEPBOX was used by a number of leading groups in the 60’s including The Beatles (who used one at Abbey Road during the recording of Revolver) The Shadows, The Animals and others.

Reverb, June 17, 2021, "The Rush PepBox: Lucy Rush on Keeping a Classic Effect Alive"

Shortly thereafter, he’d created the Rush PepBox, which was then used by the likes of The Animals, Hank Marvin, and The Beatles.

Featured in this 1966 ad. It was used on "Don't Bring Me Down", as stated in this May 15, 2001 interview with Tom Guerra.

1966 ad

Chosen as the 'greatest' by The Animals, Georgie Fame & The Blue Flames, Zoot Money, and many other leading groups.

Tom Guerra, May 15, 2001, "HILTON VALENTINE - FROM ANIMAL TO SKIFFLEDOG"

TG: “Don't Bring Me Down”?

HV: That was a 1966 Telecaster through a WEM fuzztone into the AC30. That Tele was given to me by a friend of mine, Deering Howe.

Featured in this 1966 ad.

Chosen as the 'greatest' by The Animals, Georgie Fame & The Blue Flames, Zoot Money, and many other leading groups.

A third iteration silicon WEM unit is the subject of Toop's essay "Rush Pep Box", published in Volume 17 of Leonardo Music Journal in December 2007. It is also mentioned in this March 22, 2014 Prepared Guitar interview and by Pepe Rush in this February 1, 2015 interview for Simon's Musical Den.

Leonardo Music Journal, Vol. 17, pgs. 21-23, "Rush Pep Box"

With the pep control set to its maximum output of 8, the effect was piercing, violent and lacking in sustain. Fitting the pedal with a new 9V battery opened other possibilities: the distortion could be transformed easily into a sharp click or a low pop, and passing a metal object close to the guitar pickup produced either a harsh crackle or what can best be described as tearing sounds.

At that time I was performing with drummer Paul Burwell in an improvising duo called Rain in the Face [1]. The Rush Pep Box was used in two distinct ways. The clicks, pops and crackles of the pedal were the basis for pieces that explored short percussive sounds. These were given names such as “Barbastella Barbastellus,” a reference to the echolocation clicks emitted during hunting by the Barbastelle bat, and “Barra Da Tijuca F.S. Flies Towards Ocean At Tremendous Speed,” a title pulled from a photo credit in George Adamski’s book Flying Saucers Have Landed. One of the attractions of the obviously faked photographs in Adamski’s books was the resemblance of the flying saucers to car hubcaps used by Paul Burwell as part of his percussion kit [2].

Another characteristic of the pedal, exploited in pieces such as “Cloud Studies N. 2,” recorded for our first BBC Radio Three broadcast in 1973, was its destructive interaction with other effects pedals [3]. If it were used in combination with my second fuzz box, for example, the additive sound would be a cancelled sustain. In other words, the normal decay time of the more conventional pedal would begin as usual, but then fade away into a complex, spluttering distortion or disappear abruptly. In this configuration my guitar (at that time a Fender Esquire) could sound not unlike the hybrid electronic keyboard sounds popular from the early 1960s until the mid-1970s, such as the Rocksichord, Clavinet and Clavioline. One influence on our music came from Sun Ra, and since Sun Ra had made records with these and other electronic keyboards, this increased the attraction of the effect [4].

These distinct peculiarities can be heard in the three sections of sound poet Bob Cobbing’s Judith Poem, performed by the abAna trio of Cobbing, Burwell and myself in 1973. For the first two sections, which were fast and agitated, I used the Rush Pep Box alone, then added the second fuzz box for the slower third section, activating the guitar by dragging a piece of rough timber across the strings [5].

The Rush Pep Box came to be my secret weapon in every musical situation of subsequent years. In Alterations, the multi-instrumental improvising quartet that comprised Peter Cusack, Steve Beresford, Terry Day and myself, noise frequently battled against lyricism, brutality against subtlety. I had taken to playing my guitar (by this time a Fender Telecaster) with metal plectrums or an iron bar. At high volume and in combination with other pedals, the Rush Pep Box would generate extremes of distortion and dynamic decay, closer to ripped sheet metal or exploding circuitry than an electric guitar. Chained guitar pedals will always create an unpredictable accumulation of noise, but inserting the Rush Pep Box into a chain could tip this aleatoric accretion over the edge into a force that regularly destroyed the valves in my Fender amplifier [6].

Recording solo albums such as Pink Noir (1996) [7] and Spirit World (1997) [8], I used the Rush Pep Box for the qualities detailed above, although I applied them to instruments other than guitar, such as computer-sequenced electronic keyboards or amplified flute. The distinctive crackle remained alluring and I used it in combination with Max Eastley’s Violet Ray Vitalator [9] in “Ceremony Viewed Through Iron Slit” and with Michael Prime’s recordings of bats and Robert Hampson’s shortwave radio manipulations in “Aether Talk.”

(...) Meanwhile, my Rush Pep Box continues to function after 35 years of use, generating material for both digital processing and live group improvisation.

Prepared Guitar, March 22, 2014, "David Toop 13 questions"

What's your fetish device in the sound chain?

A Wem Rush Pep Box that I’ve had since around 1970. It was designed by a man called Pepe Rush. I’ve since written an essay about him and his distortion pedal. It’s very extreme and if I’m using pedal steel guitar I can work with the magnetic field. Combined with other pedals it can be mayhem. The Beatles used it on “Tomorrow Never Knows”. Other than that it wasn’t particularly popular and I can understand why.

Simon's Musical Den, February 1, 2015, "I'm not an Italian - an interview with Pepe Rush"

I've been in touch with David Toop - he loaned me his flat box Silicon Pepbox for me to trace the circuit, but we're not using that circuit; I said I'd give him one of the new ones, he said I'll write an article on it anyway.

A third iteration unit was used for The Noise Made by People, as mentioned by Ghost Effects in this December 14, 2012 Harmony Central forum reply. It became the basis of Ghost Effects’ Silicon Pep Box (pictured together), as stated in the reply and on the official product page.

The one i do is the later large case version, silicon, and a slightly different circuit than the germanium version.

I traced the circuit from an original unit that I borrowed from Tim Felton of Broadcast/Seeland, the actual pedal is all over the 1st Broadcast album apparently.

Official Product Page

I borrowed a very rare original unit from Tim Felton (Broadcast/Seeland) to confirm the circuit and the component values for this pedal.

In a 1966 advertisement by Britishpedalcompany, the Watkins WEM Rush Pepbox is highlighted as a top choice by Colin Green, among other prominent artists like The Animals and Georgie Fame & The Blue Flames.

Mentioned by David Toop in his December 2007 essay "Rush Pep Box".

During a session at Steve Broughton’s north London studio in the 1980s, I discovered that the psychedelic noise of the Edgar Broughton Band, notorious in their time for a hippie chant entitled “Out Demons Out,” was fueled partially by the Rush Pep Box.

McDevitt is named in this Rush Pepbox advertisement from page 4 of the June 4, 1966 issue of Melody Maker.