It's a brass screw turned into the wood to prevent cracking.
It goes in deep enough to hold the forend together. But not so deep that the tip protrudes from the other side.
The head is cut off and then the wood sanded level along w/ the screw shank at the same time.
They used to lay in a fibre glass cloth epoxied into the interior to prevent the cracking which was a major issue with them.
Some of the epoxy cloth reinforcements have become loose over time.
The brass screw was likely an attempt to better that older 'fix'.
