[Electronic M]

Google a CRT type number plus the word datasheet. There are a number of tube datasheet sites that have datasheets for CRTs. Those sheets include pinouts. Of course, your tester has many more pins than a monochrome CRT the extras are so it can support a socket with 3 guns. My B&K 466 it's main connector which is a socket for the early monochrome types has much more than the 5 contacts wired into the tester, and I could easily add more wires to my universal adapter to connect to all leads of a CRT, but there is too little advantage to that in the normal use of the tester for me to bother.

Color code for color CRTs Brown=heater, black=focus (my tester don't need a connection to focus), Yellow (with R, G, or B stripe to designate which gun)=Cathode, solid color(R, G, or B)=Grid 1, White(with R, G, or B stripe to designate which gun)=G2. Many testers only need Heater, Cathode, G1 and G2 to work.
Monochrome had a different color code (which occasionally varied) I can parse one out for you if you need to know.

Look for a brightener for that 21FJP that is a good source for a base.