Most of the post-1970 production was for military use since lots of military communications equipment still called for vacuum tubes. These military tubes were given the JAN (Joint Army Navy) designation in most cases. JAN tubes come from specially selected production runs with the intent to yield durable vacuum tubes that would be able to withstand vibration and G forces common in fighter planes. Durability is certainly a nice attribute to have in a guitar amp that gets schlepped from gig to gig, night after night. Some JAN vacuum tubes are actually identical to their commercially released counterparts, which happened to be of sufficient durability for military inspectors.
Courtesy The ToneQuest