My large format camera is an older Sinar F2 works great and it has been getting some regular use of late. Just to make sure... I like to sometimes use instant film to test composition, focus and basic exposure but as most know instant film is getting difficult to come by especially in 4x5 size. I am willing to use the much more affordable smaller size Fuji FP-100c (3.25x4.25 inches) as opposed to the FP100c45 (4x5 inches). To that end I recently obtained a Polaroid 405 holder which is in good working condition compatible with the smaller size Fuji film. Film packet inserted and seem to work fine. Thought I was good to go since it mounted into the graphlock back with no problem, from what I have read thickness is sometimes an issue. The Sinar back even has some nice sliding clips to clamp it in tight and prevent light leak, but then I went to mount the back onto the camera with no luck. The film holder has a tab which projects, at the end where you pull the tab after an exposure, too far causing it to block the graphlock back with holder from seating properly as it hits against the camera standard. This prevents you from being able to close the mounting clamp required to hold the assembly into place, normally a very simple thing on the Sinar. I have tried to make it work in a variety of positions such but no such luck. If you shoot with the graphlock back mounted and the film holder improperly seated focus is off and you get light leak. Does anyone know if this is unique to the Sinar F2 but will work with other Sinar standards such as the "P"? Does anyone know if the actual "Fuji" holder, which looks like it has a slightly different geometry/shape, will work? Or am I just SOL trying to us the smaller sized more affordable film on my Sinar and if I want to shoot instant I am stuck with the overpriced 4x5 film? My Fuji PA 45 holder for the 4x5 just slides in no problems and works like a dream but since the film is discontinued and costs about 4 times the price $26/10 shots as opposed to $7/10 shots per the cheaper film is very appealing. Thanks for your expert wisdom and experience!!!