I think that the butt plates were sintered - or compressed alloy powder and definately not cast as you will see if you snap one. Easily machined too. You can polish one until the cows come home but after a day it'll just turn a dirty grey again. In the UKicon they were made by the same people who made SU Carburettors (but NOT SU) - and probably the same material too. One of the Birmingham alloy founders such as BIRMALLOY or QUALCAST