Safe to guess that seaacock sits on a tall 'backing plate' (Mound) so that the thruhull can be fully screwed into the seacock as the last step in installation. Kind of backwards from what you'ld expect. The seacock is mounted dry with the thruhull, the seacock is 'wet' mounted first, yours was probably glued in with 5200, hopefully polysulfide, left to set. Then the thruhull was screwed in as the last thing, hopefully with polysulfide. personally I would screw it in with lanoline or Dolphinite.

Thru hulls have a backing nut, which gets discarded when a seacock is installed on it. It couldn't have a nut if it had a tapered thread.

With enough rubber caulking the thruhull could fall out befor the hull would leak there, because the seacock completely covers it. And usually is screwed or bolted to the backing. The seacock has to have a female taper thread on top because that's where the plumbing begins.

Inline ball valves are exactly that, aren't they? They have no base. In line ball valves have tapered threads BOTH sides. Therefore you can't screw them down very far befor they start tightening up.

IMCO no one making inline ball valves makes a thru hull for them. Because they never were suppose to be used for under water inlets. Not only do they sit on top of the thru hull but they are not screwed very far on. And as I think Scott's photo of threads shows. you can see galling there where the mismatch has bent the threads. Tapered male to tapered female (if they are the same size) fit like socket.

I notice that Triton Tim uses inline ball cocks for above the static waterline plumbing exits. Also he has used premoulded auto hose for his cockpit drains, last I read. The reason is the tortured impossibility of bending short straight hose to compound curves. He writes a forthright description of his adventures under the cockpit - and in the lazarette.

Ron Basey in a How-To mentions nothing on the NPT, NPS anomally that Scott extracted from the marine plumbing morass. Thanks Scott.

As to Good Old Boat's two pager on "Inspecting and maintaining seacocks 101" - the drawings are great. It's ok for guys puffing on their kaywoodies with the cuffs of their white shirts turned up looking for their tapered plugs.
But as we know, there's a lot more to the understanding of saltwater seacock. I hope, without much faith, that seacocks 102 comes along that will show how to install the things, how to pull maintenance, what to look for in a questionable installation. 103 might go further into seacock design, how to recognize if your chrome balls are flaking or your stainless stell one has crevis cracking, etccc.