Just been cleaning out my crankcases before reassembling the engine when I heard a 'clink' and found this washer on the tray in my degreasing bath. The washer measures approximately 21mm OD x 14.5mm ID x 1mm thick. It was whilst I was cleaning the lower crankcase. All that is in the crankcase whilst washing was the gear drum and forks, the oil pump is removed.
Any ideas from anyone, where the washer/spacer is from please? I have drained the degreaser bath and checked for any other washers or items in there but it is clean. I have also looked through all the parts diagrams like the ones on cmsnl.com but can't find anything similar.

Hope someone can help me....
Thanks
Caley-Thistle