Probably some sealant from oil pump replacement? It's important to use sealant only where specified on engine lubrication system, since there are reports of small galley(s) getting blocked by loose, excess silicone type sealants. Remember how small the cam oiler orifices are? Smaller than .25cm. (1/10"). Clear plastic I've not seen. The reason the filter didn't grab them is they are too big to fit through the oil pump screen, so they never got to the filter?. Drain the oil (maybe thru a mesh) and check for other pieces and maybe that will give a clue as to what they are. When was oil pump changed, and why, may give other clues. If the pieces are pliable like shrinkwrap, I'd guess sealant.
The Golden Rule
1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)