if the two side edges are not flat down to the bottom but show a recess behind them, then I think Monty has it, they were called 'stripper clips' I believe, at least the ones used for the M-1 carbine.
the stack of shells had their extractor rims slid into the slot, which held them together. You would stand the stack of shells over the magazine and with one push, slide them all down into the gun and the 'clip' would slide off and remain behind, tossed aside. Anyway, if there is a slot, which looks open on the right side of the object in the picture and closed on the left, then I think Monty gets the call on this one.