Probably a miscommunication on my end, but I have an additional thought.
I have the experience of single pointing a 13 tpi when a 14 tpi was required. It did exactly this. It would thread on for ~ 1 and 1/2 threads then stop.
You may just have an odd spindle threaded to a spec so it was compatible to already owned equipment. Or the backing plate and it was forgotten about.
I first suggest you double check the tpi. on both the spindle and backing plate.
Assuming it's not a thread mismatch.
The problem as I understand you is the backing plate stops screwing on before it contacts the register. if I have this correct, it's because a backing plate for a Southbend only has to thread on .17" max until the backing plate mates with the registration face. So lets guess the threads are cut with a small margin so the backing plate will fully thread on. Say .1" so max travel is .27" before the spindle nose bottoms out at the end of the threads.
To mate with the Logan registration face, the backing plate must travel A minimum of .375" .
.365 - .27 = .095" difference and so the nose of the spindle hits the end of the threads in the backing plate and stops before the backing plate mates with the registration.
After you double check the tpi, I suggest you join the group moderated by Scott Logan and give him the measurements from your 922. I have read of some odd spindles showing up from time to time and Scott is the best factual source there is.
https://groups.io/g/Lathe-List
Lots of experience there in addition to Scott, about the only unwritten rule is because Scott sells the manuals, people generally don't post other sources.
Ron