Yes, I understand the FH is pretty rare.
The top part could be something other than aluminum but whatever it is it's not magnetic. I haven't seen the wall thickness where it penetrates the base so hadn't thought of that. Maybe it was pressed in and then machined afterwards? I know in engines they put steel sleeves in aluminum blocks so it made sense that they could put an aluminum sleeve inside steel.
Anyway.. my driver is a Westinghouse and I'm pretty sure it's original from the little I've found on the net:
I used a home-made "strap" clamp, sort of like an engine oil filter remover, to clamp on the cover and twist but the cover squeezed past the bayonet stops. With the cover off, I couldn't get a grip on the cast part of the driver and was afraid to twist on the magnet for fear of breaking it. Didn't try heat. Maybe that's the key. Not a big deal, I just restored the outside and called it good.
Ironically, I accidentally painted mine a dark brown and didn't realize it wasn't black until I got it out in the sun. I was thinking semi-gloss too but went with the gloss after asking the question here: viewtopic.php?f=7&t=306944
I do think semi gloss might look better with the horn which is semi gloss at best. Maybe one of these days I'll put a second coat on and use semi...