What is this?

[Dean Romig]

It appears that those barrels do not have the reinforced forend loop.
Presuming that the vent rib is of the "aftermarket" persuasion, and the gun may possibly have had an aftermarket BTFE.... and knowing that the non-reinforced forend loop might have pulled away from the barrels because of the extra force applied due to the BTFE, which is pretty common... (again, assuming it may have had a BTFE) that pin may have been an anchor point for a rod that went up between the barrels to the top rib to ensure the forend loop would stay put.

Just a crazy WAG on my part.

[Bill Murphy]

Assuming that is not a Parker rib, Dean and Richard are probably right about a pin to attach the forend lug to an extra piece of metal that goes into the middle of the barrels and possibly to the top rib.