Tom, the 15GP22 is different from other guns in two ways: First, it uses high voltage (around 5 kv) focus, and second, it has electrostatic convergence, with an additional element in each gun. There have been a couple of attempts to use 21AXP22 guns in 15G rebuilds, the assumption being that the screen size is so small that convergence would be adequate without dynamic convergence. I've never seen the results, though.
Black and white tube rebuilds were done with new phosphor (more expensive), or re-using the existing phosphor. This is still true today - Clinton Electronics is attempting a rebuild of the prewar 12AP4, a tube that didn't use an ion trap and therefore had ion burns. They remove the old phosphor and re-coat the tube:
http://www.earlytelevision.org/prewar_crt_rebuild.html