I've got an old Mamiya m645. It has 2 terminals on the side. One is labeled X and the other is labeled FP. What do these designations mean? Also, if I want to use a Vivitar Auto Thyristor 550 FD that has a hot shoe to PC converter (because there is no hot shoe on the Mamiya since it has a view finder that precludes the hot shoe), which terminal should I use? Also, which f stop and shutter speed should I use. I noticed there is a 60x setting and just a red dot with a circle around it on the shutter speed controller. What shutter speed should I use? The Vivitar says to use f stop 4 but this still leaves the question of what speed setting. I know this has to do with the distance of the subject. I guess I'm hoping somebody out there is familiar with the old Mamiya m645 and has used the flash terminals. I know, I know - why don't I find a user manual.