Thanks, Laitch for your help. Much appreciated.
I've search like you and didn't find it. I have part trays on my bench separated by area/task and it was in one of them upon reassembly. The last time I had it to a shop was a long while ago when it was running. It must have been 2005-6. At that time I had the splines, shaft, and clutch spline serviced as I vaguely remember. (Normally I've personally serviced my bike since 1987.) That said, I recently came across a similar-looking washer while looking at R parts at Max BMW. The only thing I can think of is the mechanic at the time might have been doing a couple of bikes at the same time and slipped it in the clutch boot area somehow. I know crazy thinking... I've been scratching my head for days. At this point, I'm not going to worry about it. It's nowhere in the diagrams for K75 bikes or in two versions of repair manuals. Leaving it to rest at this point. If I discover what it is I'll post a follow-up.
Side note:
Today I've rebuilt the rear brake upon reassembling the final drive. That's done, and I'm thankfully not under a somewhat fallen bike like a few days ago. Stupid me... Leave the door unlocked, charged phone in the front pocket, use all safety precautions going forward.
