Originally posted by Bob 256 Yes, the cast gearbox for the bellows stick out about 1/2 inch from the face of the rear end of the bellows and makes it impossible to rotate the camera (and probably even mount some cameras) using the OEM flange. In my design, I allowed a little over an inch of projection beyond the rear flange which should work nicely for rotating the camera as long as the rail isn't extended too much beyond the gear box. If, for some reason, the rail is extended, almost the same limitation applies. I can understand why they went with the shallow rear mounting flange to get the camera close to the rear opening, but some newer cameras don't handle that design well (including the K-1).
You should have a flange on the bellows side (ø 58.2mm), which secures a correct fit against the face of the bellows standard. Check the bellows unit out, you'll find the smooth ring that it fits flush to. The grove is just there to secure the mount, not for a precise fit. The groove extends to about 3.2mm from the flange, measured at the top. It is angled, maybe to 60 degrees (instead of 90 degrees). Check your locking crew, best to ask the shop to fit its angle. This way, the pressure to the flange is progressive as you tighten the locking screw. The outer diameter of the tube would be 48.5mm, the inner 43.8mm - with groves (ripples) some tenth of a millimeter deep to diffuse residual reflections in addition to the blackening. You may however just line it with some astro anti-reflection material. The groove bottom diameter is 46.4mm, it starts about 1.0mm from the flange - not overly critical but that 1mm should aid positioning.
Hope this helps to get a correct fit.
I'd strongly advise to make the adapter no longer than necessary to avoid extra strain on the rail/prism - a K-1 is very rear-heavy compared to most mechanical cameras already. My 11mm extension is plenty for the K-1 to mount cleanly. Best take your cameras and give it a try. Remember to share a picture of the result!