Looks like a wrist pin circlip. An internal circlip. Rocker shafts are likely to be outside circlips.
Wrist pin circlips can dislodged and fall easily into the
oil pan, more easily on a non-full skirt piston.
I would pull all the spark plugs and borescope all cylinders with each cylinder at BDC.
A wrist pin walking out will start to mark up the bore. A compression test of the whole engine may reveal a difference large enough to inspect a low cylinder first.