It is the frame lug.
Replacement isn't difficult as long as you take care to protect the frame.
The lug has a partially hollow pin on the back side which can be seen with the gun disassembled.
Use a drill in a hand pin vise or use a countersink to slightly cut the riveted end of the lug's riveted pin.
You want to cut just enough to remove some of the rivet to ease removal.
Resting the frame on a bench block with a hole to pass the lug, use a small punch to gently tap out the old lug out of the frame.
Install the new lug with a dab of Loctite Red and rotate the lug to the proper orientation.
Rest the lug face on a brass or covered bench block and use a larger center punch with a well-rounded point to rivet the lug pin in place.
The lug should be tight enough not to turn of loosen.
You may have to reshape the "steps" on the front face of the new lug to allow clearance for the cylinder to open unimpeded.
If you do, use cold blue to cover the bare steel.
If you'd like a better explanation and some great pictures of the process, invest a few dollars in the Jerry Kuhnhausen shop manual. This is money well spent for a S&W owner.
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