As Hack On Wheels has said, the underside of the lip and the tailcap. If you had the tailcap ano'd as well then you'll need to figure out if you want to strip the ano off of the threads on both the body and the tail cap or if you want to strip ano off of the end of the body tube and the matching area on the tail cap. The last option might be visible upon close inspection but the first option would be more of a pain to do. Also, remember that the recess in the tail cap where the spring sits needs to be bare aluminum, too.
Come to think of it, the way I would do it is to take a wood dowel of the correct diameter and glue some 400 grit sandpaper to the very end of it. Just push that inside of the body and give it a few good twists against the flat behind the end of the tube. For the threads, maybe some medium grit polishing compound on the threads then screw the tailcap on and off a few times until you get bare metal to metal contact.