I decided to try out the 19AP4.
That's a little disappointing - just a faint smudge. I guess 12kv won't cut it. The Sams doesn't mention the HV value, but I found a schematic in a Wallace Telaide that shows 17 kv.
I've contacted Moyers to see if they have a Motorola 24C792691 or Merit HVO-6 replacement. I'll do some more checking in the meantime.