Many times a hole is drilled in the bottom rib to allow easy run out of the water during a re-rust blue. That water will usually get inbetwen the ribs even from tiny pin holes in the solder joints.
It has a real hard time getting back out.
Some bbls are drilled that way from the various factorys for the same reason.
What ever the source,,it doesn't cause any problems,,there is no real need to 'patch' it up unless the looks of it bothers you.
It won't cause a rust problem.
Most Win21's have 2 of these, one just forward of the forend hanger in the bottom rib. One in the short rib betw the hanger and the bbl flats.
Done at the factory. They are usually plugged afterwards by simply staking a lead BB into the hole with a punch.
It still shows of course, but it does plug the opening,, if that matters.