Hi All, Got myself a Mamiyaflex C3 a while ago, and then purchased single exposure adaptor and film holders BUT the holders don't stay in place once slid into the back. I see from the camera manual that there should be a frame or mask that is applied on top of the holder to keep everything in place but I can't come across any of these anywhere. All the singles exposure sets I see online only come with back and film holders/slides. Does anyone have any info on this? Any help wuld be appreciated. Thanks!