I have no experience with this lock but that won’t stop me from speculating, hopefully helpfully 🙁mm):
The description sounds like the back cover is of the LOBC style (LOBC can stand for either Locked On Back Cover if the cover has an actual lock, which does not appear the be the case here, or Locked On By Combination where the combination needs to be dialed to line up the gates, but offset (e.g. on the side of the case opposite of the bolt). This will allow the cover to slide enough that it can then be lifted straight off.
What surprises me, however, is that the dial ring does not seem to have a changing index mark, which might also be the LOBC index mark. Still, this is an older lock, and perhaps the lock technician of the era simply knew to use a specific location on the dial even though it’s not clearly marked.
I hope others on this forum who have experience dealing with LOBC backs can comment on this idea. Or, perhaps you can use your borescope to see the protrusion on the back cover, and align the gates to it, rather than the lever nose.
Update: Looking at the picture more closely, there does seem to be some sort of mark on the dial ring at about the 7:00 position. Try dialing the combination to that mark rather than the opening mark at 12:00.
