Hi. Thanks for the additional info. I just tested the lens per your recommendation, and it appears to be fine and solid at infinity.
The focus screen is what I first looked at, and it too seems to be solidly in place. The focus issue has happened with 2 separate film backs, but I only have a single 120 insert to test them with. My guess is that it leaves the issue to be the insert or the mirror.
Question: When loading the film on the insert, how tightly should the film be stretched on the insert once rolled up to the starting arrow? Is some bow/arcing ok, I would think this is flattened be what appears to be two small rollers on each side of the inside of the back (top and bottom of the hole through to the shutter). Or should the rollers on the insert be pulling the film completely taught?
I'm finishing up another test roll today, then I'll load a new roll and snap a dig pic of what I'm speaking of.