Won't just masking result in a much smaller negative size? I'm not really interested on wasting film shooting substantially smaller square images for a couple extra shots per roll. If I can get it to be 16 exposures of 6x4.5, say... then it would be useful. I'd obviously rather just have a c12 or a real a-16 back, but I figured it was worth a shot... the back seems to be in great condition, other than not being a useful format for me.
You're right, just backward thinking on my part.
I do wonder why changing the front plate would change focus though, Does it affect the pressure plate?