Well Stoker gave the conclusive idea, a magnet, so simple I never thought of it. The result is non magnetic.
So, I'll come back to the boiler. My question now is the fittings, are most of them screwed in or soldered? they don't leak and they are pretty firm. I can see the gauge looks soldered and the safety valve is screwed but the other 2 fittings I'm not sure of. I'm reluctant to disturb a nice tight fitting but to make a better job of the boiler I really need the fitting removed.
I had to bite the bullet and strip completely back, due to the really bad old paint work. I then re blued the boiler to make it look as near to original as possible, it's not perfect but a big improvement.