Can't really see from the pic, but do you know how many wires come out of the other end of the lead?
Do you know if this is standard or if it's been modified? The only thing I can think of is that they have been converted to a 100v line for long cable runs. If this is the case, then the only thing you'll notice is a severe lack of volume. (you'd need a 100v line transformer for them to work - although I use the word "work" tentatively, as in general 100v systems are lacking in bass due to the inductive effect of the transformer.
Failing that, try and get the seller to open up that plug and see how it is wired. If its only two cables and two pins used, then it's just probably a seventies thing!!!