For those of you who haven't had any luck locating the elusive 19AP4, check this out. Under "Parts, etc. for sale", The 18" CRT might actually be a 19DP4, which is also a 66 degree tube like the 19AP4. It will take some ingenuity to install it, also requires additional room, as it is a fatter tube.
http://www.earlytelevision.org/class...parts_for_sale