How can it come from the crankcase?
The only way into the rocker covers from the block is through the pushrod tubes, and it looks too big to have come in by that route.
Really sorry, should have put something to scale it by! I have the heads off as a valve is leaking, and the engine on an engine stand, so when rotating the engine 90 degrees on the stand to get the first head off, this gremlin possibly could have fallen down a pushrod tube, into the cover.