Yes, the VT-231 was the military purchasing and inventory control number up until about 1945
or so. The tubes were often also marked with the JAN-xxx prefix as well. It seems that the VT-231 number was dropped around
the time the WWII effort was winding down.
But there was a lot of variation even before 1945. I have Raytheons made in 1943 which have only VT-231 marked on them,
and the same vintage only marked JAN-CRP-6SN7GT.
Raytheon is CRP, Sylvania CHS, RCA CRC, CBS-Hytron JHY, Tungsol CTL, KenRad CKR.
It's also worth knowing that there is a common misconception about those VT-231 numbered tubes. It's nothing more than
a number. If you find a 6SN7GT of the same vintage, it will be identical to the VT-231 numbered version. I have here Sylvania
6SN7GT from 1945 identical to the VT-231 numbered tube, and Raytheon 6SN7GT 1944 identical to the 1944 VT-231. Also RCA's,
Tungsols, and so on. There is no magical significance to the VT-231 number. But some people think there is, because salesmen
have told them so. It's become popular folklore in tube circles.
It's only so because VT-231 branded tubes are so common today, as the military kept thousands in warehouses collecting
dust for decades until some bureaucrat figured out there was no equipment that would ever use them. The equivalent commercial
branded tube was never kept in warehouses for decades, and in any event, most production of commercial tubes was severely
rationed to keep war production up - so there were few produced with the commercial 6SN7GT branding.
