If the hub/drum assembly can come off, there's really no need to take the drum off the hub if you just want to do the brakes. Not much different than my '51 1/2 ton front brake drums that are pretty securely fastened to the hubs with the studs as well as a couple screws.
Once those allen bolts come out, that cap should come off the axle shaft (female splines on the cap) and the bearing nut should come off, allowing the hub/drum assembly to be removed as a unit.
