They are definitely drain screws but some of mine are also bonded that way. I've noticed new seacocks have small bolts screwed into the flange and I think that's what they are for.
Be sure to measure the screw. Mine was 12-24 metric and they did not have many of them. Let me know if they are out as I have a couple of extra.
There is probably grease blocking the drain hole. With the barrel in the closed position, stick a hanger wire in there and see what happens. I had a hard time getting the new screw in until I cleaned out the threads with a tap.
question, are yours on a backing plate and bolted to the hull
? As you can see in my picture, mine are just screwed to the thru hull
witH no bolts. I Think if the hull
is an inch thick, it's not a concern. Can't imagine Bristol taking short cuts with the thru hulls. And the ply wood certainly would not have lasted 30 years.