Weird CRT needed

[toober]

OK, please forgive me, this actually not for a television - but you television guys just might have one laying about in your CRT stashes.

I am in need of an oddball CRT for a Control Data Cyber console - a
21M44P31A. It pretty much looks like an old 40s/50s era tube, but the neck
is long - really long. I do not know why. And yes, green. I suppose I
should not be too picky about the phosphor! The Cyber consoles are vector displays, used for old Cyber mainframe computers.

In the B&K CRT book, there is an entry for 21MP_ and 21M133P_, but no
21M44P_. What is the significance of the middle number in a CRT
designation? I thought originally it designated the number of electron
guns, but clearly this is not the case.