The fifth pin on the back of the connectors appears to have some kind of resistor set-up going on. I would say maybe something to do with sheilding, but then there's also a small thin wire connected, so possibly some kind of safety system for if the steppers aren't plugged in?

Power supply might be higher than 26V. There's multiple wires coming of the transformer, so without tracing wires, it's hard to say for definite.
26V AC translates to just over 36V DC, so should give a useable voltage. Won't be the best performance possible, but will still be good enough.