I did have another look at them to refresh my memory...and not mad
Steel rim (chromed and magnetic) and alloy centre (oxidised and non magnetic). A bit like Superlights in style but two piece obviously. There are lugs in the alloy with a thick bead of weld (ferrous clearly as it's rusty) between them that fixes the steel rim to the alloy centre. The weld covers what's going on, but either it's some weird welding rod they have used or there are ferrous 'tabs' cast into the alloy centre and they weld to that?
I took them in to 'world of wheels' in Qbn years back as they were the steel rim gurus at the time (Neal wheels if you remember them?) to see if they could do wider steel rims for me....they took one look and didn't want a bar of them
Anyways, pics for pointless debate by non metalurgists tomoz