I would try grasping at the two red arrows and see if it will unscrew at the blue arrow, if so you can change your batteries like this and glue your tail cap on permanently.
The reason is if he chucked up a blank he would have to have turned down on his lathe a lot of material from a large dia. blank, usually a smaller diameter blank is used for the battery tube then another shorter but wider length of bar stock for the driver junction and another length of wider copper bar for the head, if he had used just one length of bar stock and turned out the battery tube and driver knuckle it would mean starting with a longer length wide enough for the knuckle and cutting down a lot to the battery tube dia.
Chances are the battery tube and driver knuckle are 2 separate pieces that screw together, you can grip both wearing dish washing gloves to get some torque without scratching it.
Of course it is possible that the driver portion and battery tube were all turned down from just one length of bar stock and the battery spring board was installed into a lip from the top then driver half moon glued in, but I would twist the hell out of the two ends and see.
Also you can look into the battery tube from the bottom with a flashlight and see if there is a retaining ring or not, if there is then you should be able to unscrew there parts, if you can see only a retaining ring looking past your driver from the top then it went in from the top, also you could have a retainer on both sides which indicates two sections as well.