Does the boat have a cored hull
? I think today's Hunter's only core
the hull sides but in your era there might have been core
below the waterline.
If so, the plug was probably put in at manufacture to avoid the risk of water getting into the core where the sea cock penetrates the hull. This is good practice and it would make sense to use a completely rot
proof material such as fiberglass or plastic. It's good that there's no sign of water staining.
The larger rectangular area might just be a taped-off area that wasn't painted (so caulking would stick better). It's hard to tape a circle. If it is wood, poke at it to see if there's any rot
If the above guess is right, the circular plug is bonded to the inside of the fiberglass skin and the hole through the skin is just the size of the sea cock pipe. This is strong and good construction. There's no need to reinforce the edge. It would have been better if the seacock was flanged and bolted through the hull.
If the joint isn't getting larger or leaking, I really wouldn't worry about it. When the boat is next out of the water, you could sand off a little bottom paint
to see if the circle is on the outside of the hull too - this is very unlikely - and very bad.
If the hull is cored, it never hurts to drill some small shallow holes in the area (don't pierce the outer skin) and see if the drill comes out wet. If so, seek expert help. If dry, fill the holes with epoxy
and go sailing.