KBK update for Wednesday, 24th June
Having now received my order from Incotel (as mentioned in the previous update) I have now been able to examine the M76-0899 switches. These have six terminals, and I determined that two of them were functionless, until I opened one up and realised that the first switch I tested was defective. They are in fact double pole double throw (DPDT) alternate action, with each side having the opposite operation (the output terminals are NO+NC on one side, and NC+NO on the other). The mystery of M75–M78 is finally revealed: where needed, these switches have side-by-side pairs of movable contacts, with the stationary contact also split into two. The triple and quadruple pole switches work this way, as do the double-pole alternate action switches which reserve one side for the latching wheel.
In addition to being round mount, these have silver prisms for higher electrical capacity, the first time this has been observed in M7. M76-0899 is a very interesting model, that is most likely a customer-specific part.