The 15GP22 was never a practical tube, and its high cost of manufacture was a major factor in the stiff price tag on the CT100 and other sets using it. That's why the CBS 19VP22 was such an advancement in 1954. RCA immediately dropped the flat plate phosphor screen & planar mask design, and launched a crash program that resulted in the 21AXP22, with phosphors deposited directly on the inside of the bulb face and using a spherical mask like the CBS tube. (RCA traded patents with CBS for use of this design; RCA had patented a photosensitive adhesive for the phosphors that CBS needed to attach them to the bulb face. The 15GP22 has the phosphor dots silk screened on a flat glass plate).

My impression is that for a practical replacement for the 15GP22, a new design using modern guns and methods needs to be built using a compatible glass envelope. This would also require a precision yoke, designed for compatibility with CT100 circuitry, be aligned and attached at the factory. This might cost several thousand dollars per copy, but with prewar black & white sets now crossing the $10K price line, that may not be an unreasonable price in the future when all the existing 15GP22's have died.