3 that are known of, LGM (as Chris said) & the 1st Rambling Pony (on backing vocals, no bass on track) & I Believe My Time Ain't Long and there may be more. Live stuff too as Chili says, (Windsor, Marquee) though it does seem McVie joined/Bruns left closer to Sept of '67 (I think the later live dates story was extrapolated from documents Bruns had in his books of upcoming gigs he would've played at w/FM through Dec if he was still there but the Savoy Brown single Bruns was on as a SB member dates from Sept '67 so he must've still had the list but not played on the later dates but regardless of length or # of recordings, he was announced to the press/public by the band as a member. Heck, there have been bands that have had members who NEVER recorded with the bands, just live stuff etc.
As for the USC band, I asked the USC who was in the USC band when they recorded Tusk for the discog and they wouldn't tell me (for legal reasons) but undoubtedly, the USC must've had some in the USC band with short tenures but I doubt their "membership" was taken away, JMHO though.
John