The Nankai model 75 is the one I was referring too, all of the one's I found are direct copies of the New Departure model D.
There's apparently another copy out there, as one of these here is not marked Nankai, and it uses a true 3/8" 24tpi axle thread.
Not the ND size thread which is just a hair larger than 3/8" and uses only ND threaded nuts.
There are three different axle lengths, the one 20h hub had the longest axle, with just super long threads on each side, my guess was it was for adding training wheels to a kids bike. The shortest axle is the one that most closely matches an original ND model D axle.

I did find one 36h Nankai, but the chrome is rough on it. Its like new otherwise.

I tore apart three hubs and found a bunch of used internal assemblies in the box as well. None of these show any serious wear, they likely got replaced after the grease inside solidified from age. These however are likely a bit older than my buddies mid 80's Nishiki, but they are Asian, all three marked made in Japan. (Not that its worth the trouble, the two 20h hubs could easily be redrilled as 40h hubs if needed. Internally, all of these were identical, and are also identical to a New Departure Model D.

My one question is what kind of lube were these supposed to have? The ND model D had an oil port, and the bearings and clutch got greased but the discs got oiled so as not to create drag while coasting. All of these were packed with heavy black grease that had turned to what looked like road tar. I actually had to soak two of them in gasoline for a few hours to get it apart. But once it was apart, it showed almost no wear or damage.
I still have to look through my New Departure bins downstairs, there may be a few Nankai hubs mixed in there as well. (I actually found the internals of about 8 Corbin and early ND hubs mixed in with the Nankai hubs and parts).

After cleaning up all the parts, this is what's salvageable:


https://cimg9.ibsrv.net/gimg/bikefor...83dd1ccf67.jpg
https://cimg2.ibsrv.net/gimg/bikefor...4c79feee0d.jpg
https://cimg3.ibsrv.net/gimg/bikefor...b8838fefdc.jpg
https://cimg4.ibsrv.net/gimg/bikefor...439e8c2d09.jpg

A quick update here. The whole project direction has sort of changed, my buddy with the Nishiki wanted an Asian hub, but wanted something cheap, in his search for a hub, he found a whole bike for far less than a hub could be had, so the Nishiki has become my problem now. I'll likely just dig into my stash of old hubs and build up a wheel with a Bendix 70 or 76 hub. The bike he bought is a late 60's Schwinn Racer with a coaster brake and about 10 coats of house paint on it. I went through the hubs and BB for him and he's all good. It even had decent tires on it. I don't know what he paid but if he wouldn't spring for the $40 or so in parts for a hub and spokes for the other bike, I don't suppose he paid much for the Schwinn. By the looks of it I hope not.

Now, with the Nishiki in hand here, its not really a mountain bike, its a diamond frame cruiser of sorts with BMX type front forks.
He also gave me an unknown set of black chrome steel wheels that look new, they have a 126mm spaced rear hub, and both hubs are stamped steel. One option is to just tear down the black chrome rear wheel and re-lace it with a coaster brake and call it done, but I do have a few pair of older Arraya 26x1.75" '7X' rims I could also use. I think the steel rims maybe better used as balloon tire replacement rims on another project.
The frame is wide, it'll likely handle a 2.40" wide tire, maybe even wider. I stuck a 2.125" tire and wheel in there and there's an inch on both sides of the tire in the rear stays, more in the front.
I'll likely just use a set of used old school knobby tires though, I'm not paying $60 for a pair of bicycle tires. I highly doubt the bike is worth much more than that, maybe far less around here.