Have seen drums (not alloy) with inserts fitted, cant remember what for and a long time ago.
I understood they turned drum down to a fair finish then turned an insert heated drum, removed insert from freezer and fitted there were 3 drilling's for spot welds through outer drum as a fail safe.
The whole lot was then set up in lathe and turned on a mandrill to centre on bearing housing & size, it was a friends friend who was a innovator but is dead now.
He did a lot of work for vintage people, lot of work and swarf.
I had alloy drums with inserts on my Cannon trials car, I understood they were made by Alfin or Allfin. The drums were Morris Minor sized if I remember correctly. These came with the car, in bits in cardboard boxes so I don’t know the history