I have a '73 500 C/M and it has the side connector on the body, but that one does not make it possible to do TTL/OTF flash. My later 503CW has a connector that really does make it possible. I think, IIRC, that the flash connector on the side that you are referring to is a left-over from the 500C for an old flash unit that is no longer made. Just use the PC connector on the lens with a flash unit that has a sensor and can do this for you. Or, if you want, use the guide number method.