It would be a Bang & Olufsen proprietary connector. It was only a 40 watt system, even though the Beovox S55 were rated at 55w RMS. So really those speakers would be worthless, unless you are going to use them for a computer. And at over 30 years in age, I would have to say that they may not work anymore, especially if the cones & surrounds are dried out.
These are the connectors that were used on the Bang & Olufsen speakers.
http://www.oaktreevintage.com/parts/speaker/B&O_Bang-Olufsen_speaker_plugs_jacks_replacment.htm
