By the pure accident (to be completely honest I was just aimlessly browsing eBay 😀) I came across some strangely shaped GEPE slide mounts. They have 37.5 x 37.5 mm opening in the standard 50x50 form-factor and come with anti-newton glass covers.  The built-in flash popped up in my head: those mounts would be perfect for mounting the full frame slide if I ever want to project the slide with the rebate area and sprocket holes. The more utilitarian use for these mounts would be to use them to perfectly tune a scanning rig so exact  film camera 24x36 mm  dimensions would be available for framing.   Typical paper or plastic slide mounts have 23x35 mm opening to allow film frames  from the different cameras be mounted even if some cameras have the film gate of slightly varying height and width. Not only photographers have to be very precise while framing the view during the shoot,  they also will struggle with digitizing the mounted transparencies as the majority of the mounts are rather th