Track of the Wolf lists .43 Spanish Brass.
I just use Dominian and Bertram brass ... they are out there ...
Dominion is one piece brass. Made the same as any other brass.
I do the same 338 win mag, fire formed, but i use 8mm brass washers, bore out center with 13/32, remove belt, and turn down head so that the new washer will fit to a depth of 90thou. Then glue with super glue the new rim, turn down outside to .590. The new rims with extract fine, but the glue is not strong enough for sizing.