They look like rear sight bases, without the elevator, v-notch, Buckhorn, etc.
That's the dovetail insert with the spot welds.
Probably bean counter mfgr. differences.
Might check here:
One on the right is used and identical to the one that came off the gun.
One on the left is new.
Both are made for the make and model gun I have.
I'm replacing because the gun is in pretty good condition except some rust in the dovetail and the bottom side of the rear sight base that goes into the dovetail.
I'm thinking Steeltrap and Blackpowder's answer makes the most sense so I'll go with that.
Like I said it's the little things that drives me bananas