According to engineer Steve Albini, Pedal X was used for the tremolo on "Heart-Shaped Box." He also shared that Cobain tried out Pedal X for vocal overdubs and a solo on "Heart-Shaped Box", but they were ultimately scrapped, the latter at Krist Novoselic's objection. The solo can be heard in Albini's original mix of "Heart-Shaped Box", which is included on the Super Deluxe edition of In Utero.

Reddit, “steve albini AMA here is the thread” by u/steve_albini (May 30, 2020), reply to u/rock-phillistine

rock-phillistine 4. Once you mentioned he used a Sansamp, a Boss DS-2 and a Big Muff for distortion. Recently his guitar tech said he shipped two homemade fuzzes as well. Do you remember if he actually used the Big Muff or the homemade fuzzes? If so, for which tracks?

steve_albini - I only saw Kurt use regular commercial effects pedals, the ones you mentioned, and a box I brought with me that was a kind of ring modulator/overdrive called Pedal X made by a friend of mine. That might be what his tech (Ernie?) was referring to.

Caio Leme, LiveNIRVANA.com, Live Nirvana Equipment Guide, "Kurt Cobain's Amplifiers and Effects Pedals" (citing a "[p]ersonal communication" with Steve Albini)

Pedal X

First and last recording:

02/02-26/93 - Pachyderm Recording Studio, Cannon Falls, MN (In Utero sessions)

Notes:

Steve Albini recalls Kurt using a Pedal X, made by a friend of his. He describes it as "kind of a ring modulator/overdrive." (3) It was auditioned on the Heart-Shaped Box solo (28), but Krist hated it and it wasn't used in the final mix. (35)

YouTube, Al Church, "Steve Albini talks Kurt's guitars and Heart-Shaped Box I Permanent Record plays live at Pachyderm" (November 14, 2023)

Al Church: Like you said though, his pedalboard, he— it's not like he had a pedalboard, like do you—

Steve Albini: No, no, there were one or two pedals on the floor, ever.

Church: Yeah... Did you bring pedals in for him to use at all?

Albini: There was one pedal that I brought that a friend o' mine had made, Pedal X, and we actually used that on a guitar solo and on the vocal overdub. Pedal X was a, it's a kind of a brutal tremolo effect, which if you dial the speed up high enough, it becomes a ring modulator.

Church: Oh? Huh.

Albini: Like, there's also, the preamp for it has a relatively low headroom, so it drives into overdrive pretty easily, so it can be... You could use it as just an overdrive pedal, but its main function is this sort of ring modulator overdrive combination.

Church: Interesting.

Albini: And, yeah, Kurt used that for a couple o' songs for like little... neebly noisy guitar parts. And then he sang— We, he used it as an effect, like, to sing through. The... There was a... What else? Oh yeah, there was a guitar solo on... "Heart-Shaped Box" where he wanted this sort of seasick, bendy quality to be exaggerated in the... of the guitar solo, he wanted that exaggerated. And we used the Pedal X for that, but that ended up, that was one of the things, that was some contention in the band, like, Krist Novoselic hated that effect, he really didn't like it.

Church: Really?

Albini: And in the end, that song, that noisy solo, it did not get used on the final version of it.

Church: On "Heart-Shaped Box", or on the...?

Albini: Yeah.

Church: 'Kay, 'cause there is a tremolo on that, like the bowrrr.

Albini: Yeah, that's... that's the Pedal X but in a more subtle setting.

Church: Ooh, okay.

Albini: There's a version of it, s—

Church: There was a version done where that solo had like a really blistering, burning... ring modulating kind of sound and that was the, that was the version that Krist Novoselic hated.

Church: [laughs]

Albini: I just had the opportunity to talk to him last week....

Church: Oh, wow!

Albini: and, uh, I mean, he still remembered, like... "When you guys did— put that effect on that solo, it was like you put a turd in it."

Church: [laughs]

YouTube, Aaron Rash, "How Steve Albini recorded Kurt Cobain's guitars on In Utero - an in depth interview" (@ 16:21)

There was a solo in Heart-Shaped Box, yeah, where Kurt wanted a really wild sound. Like, he wanted it to be a really uncontrolled sound that would then, like, sort of snap into that sort of tremolo, that pretty tremolo. For that we used the box that my friend Mr. Bill made for me that was ring modulator overdrive pedal that he caused "Mr. [X]" or "Pedal X" and it was a ring modulator pedal that had an amplifier that could be overdriven in it. At its lowest settings it was a tremolo, at its highest settings it was a ring modulator and anywhere along that path you could turn the gain up and it would overdrive. But it only had an on/off switch, it wasn't like you could turn off the tremolo and then turn on the gain – it was always one or the other, you know. It was always on or always off... and Kurt played around with it for a while and he came up with this really brittle ratchety sound for that solo and he loved it. And then when he played it for Krist, Krist absolutely hated it. He was like, "Why are you putting this big smelly turd in the middle of this great song?" Like, he really thought Kurt was trying to sabotage a beautiful song. Like, "Oh you think you've got a hit here? Here, let me fuck this up for you so you can't play it on the radio or whatever." And he thought that was like a strategy of Kurt's. I'm certain it wasn't. He was just— Kurt was just enjoying this freak sound.