While I was out today with my Mamiya Universal I noticed that the trigger on the Type 3 back felt a little stiffer than usual, but I pressed it anyway. I noticed this pin, pictured below, fall out. It looks like it has fallen from this spot, which triggers the lever above. I can no longer trip the shutter with either the Type 3 back or trigger on the hand grip.
I'm not entirely sure if I've broken something, of it's come loose and can be easily repaired.