The photo is cropped so here is the other end.

Doesn't look B&O to me, but Martin is probably the person to ask...

I don't know what it is.
It seems to be hollow - what's inside?

Martin 

It might not be B&O, I was just looking again at the quality of the 4 pins at the bayonet end, they look quite basic. As for whats inside, no idea, it is sealed, has the weight/size ratio of a cartridge. I posted the question on the vinyl engine, maybe someone there has an idea.

Thanks.

Some kind of measuring tool, perhaps.

Got a result, 

I did contact the previous owner. He had it in his attic for 35 years and never used it or knew what it was. Someone on the vinyl engine suggested a SME adapter for B&O, further searching revealed SME produced adapters for Decca heads. D.2 and D.3, I think the D.2 is stereo and D.3 mono. Need to do further research.

Thanks for the replies, 

Regards, 

Mark.

Might be worth a couple of quid! ;-)

http://www.audioscope.net/decca-d2-sme-adaptor-p-2552.html

Might well be, the Decca enthusiasts are seriously advanced at modding stuff. Hopefully a Japanese gentleman that likes mono is looking for one. That German site is expensive though. Or maybe the rest of us are too cheap. 

Might well be. Those Decca enthusiasts need deep pockets,. Hopefully there is a Japanese gentleman that has a collection of mono heads after one. That site is expensive though. Always get that popping up when searching for stuff.