It’s the crush washer off of the cam chain tensioner mount
I don’t know where it was caught, behind the cam chain guide maybe, but after I’d pulled the barrel off it appeared on the engine casing.
So, the crank turns lovely now, new gaskets etc arriving tomorrow.
Result
We could learn a lot from crayons; some are sharp, some are pretty, some are dull, while others bright, some have weird names, but they all have learned to live together in the same box.”