It's a 4mm Serval. I pulled it apart this morning. I could clearly see the washers on the clutch shaft, but couldn't on the sprocket shaft.
So I tore the sprocket shaft gears off and everything was there.
I don't know what the fuck that piece was or where it came from, but it wasn't from that transmission.
I am guessing it was left over from a rebuild.
I put in new crank seals, the clutch shaft retaining washer and a new idler gear bushing and reassembled.
Ready to go in an hour and a half.