Not all THAT light, IMO. Just a few pounds per side lighter.
Also available from US sources, like TRF.
And with respect to TR4Tony, the ad is rather misleading. The same drum does NOT fit TR3-4 and TR4A-6 properly. The solid axle cars need larger stud holes and more trimming to clear the backplate.
Besides, TR3s were never fitted with 9" Girling brakes, even though the ad implies they were. The change to 9" Girling didn't happen until well into the 3A series.