They are probably mono headphones, if the plugs you say are just prongs, and not multicontact connectors. Anyway, they should work just fine in any Beomaster headphone output with an adapter, which you most probably have to build yourself.
The old headphone plugs usually were banana type connectors. You should connect one of them (preferably black, it might be connected to the headphone "chassis") to the sleeve part of a normal stereo headphone plug, and the other prong both to the ring and tip of the same plug.