[old_tv_nut]

The following article indicates the ZnS:Ag blue was used in the 21AXP22 from the beginning.

DEVELOPMENT OF A 21-INCH METAL-ENVELOPE
COLOR KINESCOPE*
By
H. R. Seelen, H. C. Moodey, D. D. VanOrmer, and A. M. Morrell
RCA Tube Division,
Lancaster, Pa.

Fink's telvision Engineering Handbook 1957, p. 1-35, has a list of phosphor coordinates with two blues:

Blue silicate phosphor x=0.163, y=0.126 This must be the one meant to avoid copper contamination. It is much more toward cyan than the NTSC spec.

Blue zinc sulfide x= 0.141, y= 0.082, which is exactly the NTSC spec.

Modern blue zinc sulfide (sRGB, PAL, SMPTE, HDTV) is x=0.15, y=0.06

This has me wondering if 15GP22's have the sulfide and the copper contamination referred to earlier experimental tubes. Would have to put a spectrometer on some tubes to settle it.