My guess is those drums are probably lined with a mild rust inhibitor. If they weren't treated with something, they'd rust up within a week. That type of a treatment is very easy to remove, I'd just burn it out. There are probably other ways to remove the thin coating in case you want to preserve the original paint job.
Another thing to think about when choosing between open head and closed head drums. The open head drums tend to have a larger top lip, which makes adding a weber lid a bit more difficult. Many people have cut the bottoms out of open head drums, mounted the flat lid to bottom and use the bottom for the top. If that makes sense.
Even starting with a closed head drum, I still had to modify the el cheapo kettle lid to fit on top edge nicely. The lid was just a smidge too small, so I had to pound out the lip of lid a bit.
Modded Brinkmann Gourmet. Basic UDS. Rescued 22.5" OTS. SJS Mini-WSM. Stubbs. B&B Oak Lump.
Cannondale 2009 F8 (for burning off all the great BBQ)