Usually they will have steel weights spot welded to outside of the drum if they need balancing.

If it is truly a lead weight glued to it I would not expect the glue to last long with the heat that can be generated.

So, yes, it's not unusual for a drum to be balanced with an external weight.
If it is welded on it's supposed to be there.
I it is glued on then take it back, as it won't last.

If you want to find out, use the propane torch on it, it either sticks or falls off. The drum could even get hot enough to melt lead.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-