That's curious that the nut would stop, basically the combo thread is the same all the way down but tapered on top, it should be loose then fill in instead of stopping like that. I know you're past that point but it looks like you may have one of the non standard foreign threads. I've looked and cant find the "combination" threaded fittings where do you get them?
Agree on the thread issue. The pitch and thread angle may be a little off. On the tapered end, there is enough "play" between the threads that the difference allows the lock nut to go on, but once it hits the parallel threads, that is now longer the case. I do not think the "combination thread" thru hulls are commercially available. They look to be original to my boat that was built in Australia in 1978. The fittings are commercial thru hulls, but I imagine the builder then machined a taper so that standard ball valves (with female taper pipe thread) could be fitted.
Here is some information on "Combination Threads" from Jamestown Marine:
Installing Thru-Hull Fittings and Valves - Groco
IN-LINE VALVE INSTALLATION
GROCO does not recommend the use of in-line valves as seacocks for these reasons:
An in-line valve has no means of attachment to the vessel hull or backing block, so the valve can turn or loosen from the thru-hull fitting with vibration or with normal use.
If the connected thru-hull fitting becomes damaged or broken, as might occur if the vessel struck a submerged or floating object, or if the fitting was inadvertently damaged or broken inside the hull, there would be no way to shut off the flow of water into the vessel.
In-line valves have NPT threads, which are not compatible with NPS threaded thru-hull fittings (unless the thru-hull fitting is machined with "Combination Thread". Installing an in-line valve onto a thru-hull fitting will create a mismatch of threads resulting in minimal thread engagement between valve and fitting, and an unsafe installation. Property damage, personal injury, or both could occur. If you choose to utilize an in-line valve as a seacock, the thru-hull fitting used must have "Combination Thread".
FIGURE-2 shows a sample installation of an in-line valve used as a seacock.
STEP-1: Choose a location that is out of the way of foot traffic.
STEP-2: Cut a hole through the hull and backing block slightly larger than the thru-hull fitting OD.
STEP-3: Insert the thru-hull into the hole and tighten the lock nut.
STEP-4: Apply TFE thread tape to the thru-hull fitting and screw on the in-line valve securely. Use a thru-hull installation tool (GROCO THT-530) to hold the thru-hull during valve tightening. If the distance between the lock nut and the inline valve exceeds 1/2", remove the valve and thru-hull lock nut, and use a thicker backing block.
Non-standard pipe threads on a thruhull fitting comprised of NPT threads at the lead end, blending smoothly into NPS threads for the remainder of the fitting to the flange end.
A flanged NPS-threaded fitting of sufficient length to penetrate hulls of various thickness, and to accept a seacock having NPS threads. The flange is usually grooved to accept caulking material, and is drawn tight against the outside of the hull by the fastening of the seacock inside the hull. A thru-hull fitting is for above or below waterline use, and may be machined with "Combination Thread".
Here is a clearer PDF:
Here is one way to get a taper thread on a straight thread thru hull: