Thanks to local hero @theprofessor, I rediscovered the article by Ludwell Sibley about RCA date codes. A couple of years ago, I cleaned up. RCA consumer tubes from the mid 50s on usually have a 2 letter GE datecodes are tougher to work with as there's not a handy chart like the.
Sylvania was a major manufacturer of Military tubes and selected a 3 digit lettering system to designate Sylvania products. One system prefixes a three-digit number with the letters "VT", presumably meaning "Vacuum Tube".
Founded in the early s as a business that renewed burned-out light bulbs, Sylvania and its predecessor companies ultimately began producing new lamps and then vacuum tubes for the radio industry. Especially, during the 's war effort, it was a common practice for USA factories to source tubes from each other and re-label them.