I just built two using a barrel just like that for friends. That appears to just be the rust inhibitor, which all have (if they aren't otherwise lined). A burnout will make it so that you can just sand the soot out. My personal uds has ribs, but these last two without ribs look sharper. The ribs are there so that fully loaded (500 lbs) drums can be laid on their sides and rolled to transport them. The lack of them will not cause you any issues at all. Some companies need them without ribs for removing the product. For example, some adhesive comes in straight sided barrels. The company turns them upside down, lightly heats the product, and the chunk of whatever is in there cleanly comes out.
Hope this helps.