They're little semi-circular pieces that go around the top of the valve stem to hold the valve spring around the valve. If the keepers came loose, you should see a valve with a spring sticking out beyond the stem. I suppose it's also possible that someone replaced a valve spring and inadvertently left some keepers on top of the head which eventually fell into the chain cavity and got mixed up with the chain.

Have you rotated the chain thru a whole cycle yet and taken a close look at all the links?

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13