Ok, so I decided to replace the roller bearings on the two transmission shafts as well as I didn't fancy pulling it all apart again because I replaced the wrong bearing!
However, when re-assembling the countershaft, I have ended up with a spare thrust washer. I've checked and rechecked the parts book and everything is assembled as per the diagram. My suspicion is that the PO may have added it for some reason (it was a bit clunky to change the gears).
Additionally, the left over washer looks home made as the center hole is not circular - see pic attached.
My question is, is the parts diagram correct or is there another washer, and if so, where does it go ?
This 'washer' may be a leftover from around 1974 when one of the motorcycle "tech" columns addressed someone's concerns about his CB500 losing clutch lift at the clutch lever. This particular genius magazine writer decided that the mainshaft has enough "play" in it that the clutch was "...running away from the lifter rod..." (or something along those lines) and he was completely sure that Honda hadn't forseen this eventuality when designing this engine (despite the fact that the design came from the uber-successful CB/CL350 and CB/CL72/77 and CB450 twins). Then he recommended "shimming the clutch away from the cases" with copper or crushable washers so it wouldn't run away from the lifter rod.
The real problem was that the owner had never lubed his clutch lifter even once, and it had a serious groove worn into it: I fixed the one many times on both the 500 and the twins. If that washer came off the mainshaft from behind the clutch, that would be my suspicion.
