Writeup by /u/Z3FM on Reddit:

There's a part in there that's a bit simplified and not exactly true. SMPTE-C actually was created to address the disparity between the original CIE 1953 NTSC transmission standard and the drifting NTSC TV outputs that deviated over time for manufacturers to compete in the marketplace. Details here.

It was the original NTSC standard that tried in vain (because of limited phosphor and tube tech) to tackle the film gamut with its relatively large colorspace. SMPTE-C was the first attempt to reign it in, and P22 was the definition of the primaries, named after the P22 phosphor that could meet those criteria.

SMPTE-C (1994) as we know it in this context is a transitional color space that was devised with (then) modern digital HD and PC colorspaces in mind, and also moving on from the colorspace of analog HD, while folding in the original NTSC colorspace with adjustments. It's the colorspace still in use today, part of rec.709, which is the US HD broadcast standard. The colorspace that actually can try to tackle film gamut again is rec.2020 and rec.2100.

SMPTE-C phosphors (edit: which is just a name, it still used P22 but tweaked) were made to better tolerances and shorter persistence than their other P22 counterparts, which is why they were often used in high TVL and progressive scan models for...yep, HD playback, which had to conform to SMPTE-C.

tl;dr/don't care: So if you are comparing P22 to SMPTE-C, SMPTE-C tubes will have the faster response time in persistence, and if you desire more accurate color rendition, it will slightly edge out P22 in certain situations and workflows. Also, SMPTE-C monitors were often relegated to 800 line tubes because they could be dual-purpose progscan tubes due to their faster response. (edit: talking about SD monitors here. PC CRTs were using high-end P22 akin to SMPTE-C)

But that last point was not always the case. Strangely, I've found SMPTE-C tubes in some monitors that would never be in a situation to use them as originally intended.

SMPTE C was a colorimetry attempt by Conrac Inc and RCA in 1968 to set RGB primaries using P22 phosphors to address the problems that TV makers were encountering to reasonably constrain the ambitious original NTSC standard. Because only like 1 or 2 TVs could attempt to reproduce NTSC 1953, and they were pretty bad displays by our standards.

If monitors say SMPTE-C, it meant that that monitor could adhere to that standard and was also made to higher tolerances and standards to achieve it for broadcast needs. You WILL get the SMPTE C NTSC colors if you get that monitor. If you get a P22 monitor, it is not a guarantee of SMPTE C conformity, but it is possible if the monitor is high quality. SMPTE C is there as a designation so people with mission-critical situations don't have to guess.

Phosphor designations can lead to a lot of confusion, since under P22 there were about 7 different variations in the Zinc-Sulfide formulations to create the desired cathodoluminescence, with varying persistence decay.

B22 is the same phosphor as P22. P22 was the EIA (US) designation and B22 was the EIAJ/JIS (Japanese) designation. That PC CRT manufacturers would use B22 in their promotional materials to describe the tube technology was marketing BS.

The later medium-short persistence formulations that came from P22 are what we see in the Diamondtron, but since it's a Japanese company with a Japanese tube, they had full license to call them B22 tubes to make them appear unique.