Help identifying a pin/medallion/coin?

[Biber]

I'm trying to identify an item in a box of medals at Yad Vashem that belonged to Moshe Gildenman (Jewish partisan turned Soviet partisan turned Red Army officer). I'm not sure what it is. There isn't a ribbon ring and seemingly nothing to suggest there's a pin on the back side, So it might even be a coin.

It's the round silver item next to the Polish Partisan Cross and partially beneath the Soviet partisan medal. The second image is zoomed in and rotated.

Does anybody recognize it by chance? I'll post this in the Polish forum too.