Sealing SeaCocks

[ebb]

Can't say what that is. Like looking at a galaxy, but not knowing what part of the sky it's in!

Most repairs to former holes are made to look as if the holes were never there....almost.
A squared off 'plate' like you have there is unusual.
Most plates are included in a new thru-hull instalation to beef up the immediated area around the hole.
Most back-up plates are circular, because that shape is easiest to fit to the curves of the hull.

What is it doing there? Covering up a circular hole, as a fix? Or covering up some damage of some sort? In terms of fiberglass repair, it is strange.
I would poke it with an icepick to see if it moves! Of course, what I mean is to see what the nebula (thing) is glued on with.
The runny white stuff could be 2-part plastic - either polyester or epoxy. If it's hard, it's plastic, rather than rubber, say. When you abrade the material at the join of the plate and the hull (what the plate seems to be glued on with) and it SMELLS sweet.... it could be polyester.
Polyester is a clue that it could be original. However, some blow-hard experts have been known to advise skippers to repair their boats with polyester....

polyester is not a glue (epoxy is a glue).
Former owner may have attempted a fix with polyester, sure hope not....because you will have to fix the fix!!
Polyester is the material used to construct the hull. At the time of building Pearson was able to glue and encapsulate on to the interior of the hulll various strips of wood (stringers, cleats) to attach furniture and cabinets (settees and lockers) to fresh polyester.
For instance, our Ariel has long pieces of wood attachted to the hull in the cabin that the shelves (shelves) rest on.
This structural member is reinforced with fiberglass, and may not look like a strip of wood anymore. They used fiberglass batting (mat) to build-up, glue and cover (encapsulate) wood to the hull. Plywood bulkheads are Tabbed to the hull with fiberglass cloth or roving.

In areas of the boat that are out of sight, no paint was used to cover construction by the factory.
While that creature in your photo looks like a former owner's contribution (because it's maybe funkier than what Pearson did), it could possibly be something original with the boat.
My Ariel #338 came with a porto-pottie, Had no hard plumbing for the head.
Don't know the history of our boats like Commador Bill....but some models obviously had wet plumbing, and yours (as evidenced by your thru-hull photo) is one of them. Could still be a former owner's addition.
See if you can find out if that rectangular piece of whatever is covering over something. I don't think so...
Could it just be a piece of wood used to hold something? Like a prop or stand-off for the head's waste holding tank in the V-berth? OR a fresh water tank. Maybe one or both were removed?

50 year old boat...anything is possible.
Back your photos off a bit to give the object in question a bit of reference to what's around it...
Like they say in real estate: location, location, location.

Somebody else here on the board will know for sure.