Logic has no room in the automotive re-manufacturing world! Rebuilders get TONS of used tcases, etc from junkyards. Many are just used units with say a hole in the case. They put the internals into another case and Voilà! rebuilt tcase, please pay on your way out!
The same VC is used in NP119, 129, 219 and 229 (possibly some others) The FSJ NP219 is a direct swap for the NP119 except it also has low range (an added benefit) and E drive. They were only used '80 to mid '82 (Jeep also used NP208 in a lot of those rigs) With the number of FSJs "retiring" and not needing parts, the number of spare parts being available is starting to dwindle too.
I've been in the automotive repair field for nearly 40 years. The number of unethical repair shops used to hover around the 80% mark and I don't think it has changed that much over the years. I sell parts now and the number of "shops" that call to pick my brain about possible issues with pre-OBDII vehicles is close to 80%. They are what we call the "Plug and Prey" mechanics that I wouldn't take anything but the most menial tasks to.
The VC is one of those "Works or doesn't work" things that's easy to test visually if it's in your hand. Some rebuild shops even swap the VC for the differential from a NP128 or NP228.
I agree it would be illogical to use a 30 year old VC however many large companies don't run off of logic. They run off of dollars. If they can save a few bucks and reuse a part that still works they will. I'm not talking about gaskets and Orings. I knew a guy who worked for a major engine rebuilder. They would routinely overbore one cylinder and and just hone the rest if they were in spec. So you would get an engine with one or two .030 over pistons and the rest are cleaned originals. I'm not saying that's what they do with the TC. That's why I said to call them.
Eaglefreek that would be an extremely unusual situation, no reputable engine builder would do that. And certainly it would be far from routine for an engine to come in for a rebuild with one cylinder out of spec and the other's in spec so the odds of getting a rebuilt engine with used pistons from an unscrupulous manufacturer would be very slim in any event.
As I said earlier in order for them to use used VC's in spec they'd have to find them first and either every core that comes in would have to have an in spec VC (impossible) or they'd have to go out and buy 10 or 20 used transfer cases to find one good in spec VC, that would be time consuming and more expensive than putting in new VCs if it could even be done. If you're saying its possible they could put in used VC's sure anything is possible, they could be getting in cores, cleaning the exterior and sending them out as rebuilt units. The question is is it likely and common sense says no, its not likely at all.
Now if you don't have any suggestions as to where I can find a new viscous coupling at a price proportional to the $620 Autozone sells a remanufactured transfer case for I have no need of further comments from you.