It's more like a raised ring or collar that goes around the hub a couple inches inward from the end of the hub. Or sometimes it's just the raised end portion of the hub that is conical in shape, sloping towards the end of the hub. In both cases, the puller snugs up behind that raised edge and pulls against it when the screw is turned against the axle.
Yes, a reversed axle nut should work in place of the missing "buffer" cap" when using the puller you have. That will keep the end of the axle shaft from mushrooming when pressure is applied. As the hub and drum work their way outwards and not as much pressure is needed to be exerted against the tip of the axle, you may need to remove the nut to finish turning the screw so that the hub and drum can be completely removed. Otherwise, the screw will bottom out against the nut and not allow further tightening. Usually, though, by that time, you should be able to pull the drum off the rest of the way by hand. Usually.
Be prepared: you MAY have to also heat the hub in order to get it to budge. Sometimes they can be REAL b*stards to remove, especially if the chassis has sat outside in the elements for a long time.
Marshall