Ha ha, yes, the Can't-Remember-****-itus.
I think I know what happened. I kept all eight old pistons laying on my work bench for a long time before finally chucking them in the bin. One of the new ones might have slipped into that pile while I was trying to get the pistons in last time.
But I guess it might still turn up somehow. The quickest way to find it is probably to buy a new one ...