D3a Tube Question

Could someone please explain what "Post Gabl" means when etched on a D3a output tube? I saw some NOS D3a's on eBay without this etching, yet my current tube has the "post gabl" etching.

Something to do with government use.

However I've never seen "post" used with "gabl" (usually 3 letters and one is a "b" - don't recall the rest).

I may be mistaken about the "3 letters" and confusing that with something else, but I'm certain that I once translated a page in German (to English) to find the government connection of GABL.

Tried Googling for it but came up with zip.