104.980 part identification

[jamminrose]

I am putting back together my 1990 300ce 104.980 that I did not take apart. I have a part that at first glance looks like a wood-ruff key with one end ground off and a notch on the flat side. Its approximate size is 10mm long, 5mm tall and 5mm thick. I had to rescue my car from a mechanic gone bad and ended up with a box of parts and a half assembled engine. It is possible that the part fell into the box at his shop, but I do not think so. I can almost complete assembly and see no place where this part could go.