hi Dafydd, sorry for the bad news, the part is discontinued and there are no sources. if you find a reproduction source for this part many people would be interested to get some of these drums.
i think it's possible to swap out all the brake parts for the 3/4 ton 12 inch drum onto your existing axle but i haven't had to resort to that yet. please keep us updated on your progress best of luck -stan
Hy Chevydafz1, welcome, unfortunately as 2ManyTrucks has said those drums which are only supposed to be 14" inside diameter have been made of unobtainium for years. They were used up to 1959, in 1960 the drums were changed to 13" inside diameter which is much more common, if you could find an early 60s one ton you could get the drums and backing plate and convert to the newer drums. I know it's a long shot where you are, but it is an option.