I've been harping on this subject for a long time. The drums are NOT balanced from the factory and there is little point in balancing wheels under this condition. Buying new drums is a crapshoot because they may be even worse than the ones you are replacing. The OP is the first person that has come on here having actually tested the balance of the drums.
OP, you are correct that there is no provision for balancing. What I do is clean out ALL the grease from the bearings, add a few drops of light oil, and find the heavy spot. Then mount the tire/wheel with the heavy spot opposite the drum heavy spot. Mark everything and add balance weights to the wheel till it comes into balance. Clean the bearings, pack grease and reassemble with a new grease seal. Using this procedure, you should be able to get the entire assemblies within 1/2 oz. If you just balance the tire, you could be out 6 oz. or more.
