According to Rumours producer Ken Caillat, an ECM-50 was used to record guitar on "Dreams" and "Silver Spring". This is stated in his book Making Rumours: The Inside Story of the Classic Fleetwood Mac Album and this August 2007 Sound on Sound interview, respectively.

Making Rumours: The Inside Story of the Classic Fleetwood Mac Album (April 10, 2012)

I opened up the mics, a Neuman 414, an AKG C-451, panned hard left and right, and my little Sony ECM-50, taped to the front of Lindsey’s fine guitar.

Sound on Sound, August 2007

"Take 'Silver Springs' [the B-side of 'Go Your Own Way']. I taped this Sony ECM50 lavalier mic onto Lindsey's Fender Strat, which was kind of a crazy idea because no sound would be coming out of there. However, I noticed, when he would sit around and play in the studio, that I liked the sound of the high frequency that comes off the strings — it's hardly a note, but more of a second-octave, third-octave harmonic thing. So I taped the ECM50 on there and he was actually playing the part through his volume pedal, meaning that when he plucked the string and opened up the pedal you'd hear this 'wah' sound', while preceding that there would be the little glassy clink of the ECM50. Then we ran the pedal sound through the Leslie and had a delay on that, slowing his part down — he was actually going to double that part, but then when he heard the delay he started playing along to it and that changed the whole tempo of the song... You wouldn't have had that in the Pro Tools world, where there's no credibility given to putting some space into the songs. Back then, you'd put echo on there and create space, and you were painting a portrait while you were going."

In Vol. 4, No. 5 (the September/October 1973 issue) of Recording Engineer / Producer (a.k.a. R-e/p) magazine, an interview with MGM Recording's Ed Greene mentions that Sinatra used the Sony ECM-50 for part of Magnavox Presents Frank Sinatra, which promoted the album Ol' Blue Eyes Is Back.

The hand microphone that we ended up with was a Neumann KM -83, believe it or not, an omni microphone. I hated to use the word "omni" around people who weren't really technically oriented because right away they said, "Omigod, that'll never work!" But, in fact, it worked very well. We must have tried 15 or 18 microphones and came down to about 5 that we tried on the show. I originally shied away myself from an "omni" just because you have to work it a little closer. But in this case, being the professional that he is, Mr. Sinatra worked it just beautifully. On the segment of the show that required a lavalier we used an electret Sony ECM-50, also an omni, because its sound closely matched the KM-83.