I am not seeing any piece... or anything for that matter.
But, if it is a piece of mostly flat metal about 3/8" wide with about a 1/4" hole in it it is a shell latch that goes in the receiver. They are supposed to be staked in place, but when the gun is correctly asembled they can't move anyway. Most decent gunsmiths have the special tool needed to re-stake them in place. Or you can do what I did; Superglue. I did it "temporarily" about 4 years ago, now I don't remember which one it was.
Those parts are not staked in the 11/48, as a rule, but are held in place manually while fitting the trigger plate assembly into position.
The later models have those similar parts staked in, but the guns with a side button carrier release don't use stakings.
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