I've got the camera disassembled which has made it much easier to see how it's designed and how I can go about fitting a second back to it with correct alignment and registration.
The camera was designed with an internal offset that allowed attachment of the back on the left and accommodated the original ground glass on the right. This offset actually made it much easier to determine how to change the seating of the original back to allow the second to sit in the same position on the other side. After making a couple of measurements to verify that they would seat in equivalent positions on each side, I cut a couple of grooves to allow the backs to seat flat (the backs have bent edges which reach around the edges of the body so a couple of grooves were needed so the the inside ridges weren't raised by the edges of the body). Once those grooves were cut, the backs seated exactly as I'd hoped.
This also showed me how much the outer body needs to be extended to meet the overhang of the far left and far right edges of the back assemblies mounting plates. I'll need to make a couple of wood wedge shaped pieces to (or possibly just blocks depending on aesthetics and my woodworking capabilities with the minimal tools I have) to extend out the sides of the body on both sides.
Finally, to get both backs to seat properly butted up against each other while seated in the new grooves, I needed to remove the button that releases the revolving back on the right side back assembly along with its spring and part of the internal arm that the button would engage when releasing the lock that allows the back to revolve. The below image shows a comparison with one back retaining the button and the other removed.
Once that was taken care of, the backs were able to seat butted up edge to edge, sharing the new grooves, and in proper alignment.
