On a steel framed revolver, there is a pin set in the rear, its hole is drilled into the joint between the arbor and the frame. You must drill this pin out with a proper sized drill, preferably a tap drill for a #6 set screw. With the pin drilled out, the arbor can be unscrewed. See the exploded views on the VTI website, the pin is the same part used to locate the barrel. (Uberti part no 23).
When the (same) arbor is reinserted, tap the hole for a #6-32 set screw and lock it in with that. Future disassembly will be much easier.
If the arbor is replaced with a new part, you will have to complete the half-hole in it after the wedge slot is lined up, by marking it and using a 7/64 ball end mill or similar tool to cut a trench in the end of the arbor to form its half of the hole for the set screw. If your end mill is rigid enough, you could use it as a drill into the same hole, completing the arbor's half of the hole.
It might be safer to mill a groove using a ball end mill with the arbor removed from the frame, but it must be correctly indexed to maintain the wedge slot alignment. Also needs to be at the correct depth, as a groove, to maintain the hole's proper diameter for the #6 tap, when assembled.
A brass framed revolver often has no locking pin, as it's too hard to drill a hole on the joint between two metals so different in hardness.
