[JohnAdams]

RCA would be "EIA 274".

I looked at several things I have around and they don't use the word "code", just "EIA 343" for Zenith. (but their CRT's were EIA 1101 for the (Rauland division), Zenith company.
CRT's, tubes, modules and some other parts usually had the EIA code on them.
In the 60's it seemed like a lot of parts were shared. Some receiving tubes in Zenith would have "Zenith" printed on them but the EIA 274 would tell you it was a RCA tube. I think it got to the point that there was no reason for 3 or 4 companies to build the same receiving tube, so one company would build all of 1 type and they would share. With the EIA code you could always tell who was responsible if a problem developed. One of my Zeniths has an original CRT that although labeled "Zenith" was built by Sylvania. I even saw a few new sets come with a crt built by Channel Master.

Just curious what went in the DuMont.

Here is a partial list of EIA codes someone posted.
Tubes and CRT's EIA code
Amperex (USA) 111
Bendix 125
DuMont 158
Eimac (Eitel-McCullough, Inc) 162
Electronic Tube Corp 169
General Electric Co (USA) 188
Hytron (CBS-Hytron) 210
Machlett 231
RCA (Radio Corp of America) 274
Raytheon 280
Superior Tube Co 310
Sylvania (Hygrade Sylvania Corp) 312
Tung-Sol 322
United Electronics 323
Western Electric 336
Westinghouse 337
Zenith Radio Corp (CRT's) 343 (Rauland division 1101)
Nortn American Philips Corp 423
Taylor (aka Cetron-Taylor) 713
Lewis & Kaufman 738
National Electronics (also Cetron) 749
Penta Laboratories 771
Vacuum Tube Products 781
Varian Associates 809
Litton Industries 879
Electrons, Inc 935