Hi Jack,
I adapted a Canon-clone timer/intervalometer/remote release to work on my Mamiya 645AFD, and posted a description of this here a few months ago.
I don't think you'll find the connector as a part
on its own anywhere. Your best bet is probably to find a cable which already has one of these connectors; perhaps used or new-old-stock. Mamiya made 2 types of manual switch cables (long and straight; short and spiral). My source was a Quantum Radio Motor Drive Cord #467, like
this one.
It had the advantage of already having a 2.5mm jack at the other end of its spiral cable. The fact that the jack was 2-pole (mono, if it were audio) told me all I needed to know: the Mamiya bodies are triggered simply by closing a switch, completing a circuit between the 2 poles of the jack. This implied that only 2 of the 8 pins on the body/connector mattered as well. By a little trial and error, I found that these are
the 2 pins at 8 o'clock and 10 o'clock on the body socket - the ones drawn with a * below:
0 0
* 0
* 0
0 0
If you just have a Mamiya body, get a piece of wire and hold it to these 2 pins; this closes the circuit I mentioned, just as pressing the normal shutter release does. That little piece of wire will trip the shutter at whatever speed the body has been set to, including the Av/Tv/P modes.
Good luck with your project! Keep us posted as to progress.
Ray