21AXP22 is the original 21 inch round color tube with a metal cone. 21FJP22 is a much later generation round all-glass tube, which includes incremental improvements such as improved brightness phosphors (of several types over the years, culminating in tubes with rare-earth red) and other changes such as better color purity. The metal-cone tubes were also prone to develop leaks at the seals between glass and metal or metal to metal, which were non-existent with all-glass construction.
The 21FBP22 was preceded for a brief time by the all-glass 21CYP22.
A paper on the 21CYP22 improvements is at:
http://www.earlytelevision.org/pdf/d...eview_9-55.pdf