6SN7 suffixes

Fatparrot,
You can also go over to vintagetubservices.com and make your way to the stock list page, then click on octal tubes.

If you read your way down thru the selections and brands,
you will also get a minor description and history of the
differant types.

A proper answer to your question would require a short term
paper on 6SN7's as it would cover the history, require-ments, uses, tooling changes, etc... thru the years. But
Andy puts in some helpfull, and interesting tidbits in the
price list that may help you out.

Fatparrot, when I first got into the 6SN7s I did exactly what Charliee is suggesting. Andy has breakdowns of all the tubes he carries with some nice brief descriptions.

Early 6SN7 had metal cases. the GT means "Glass Tube".

Thanks all! Charliee, I use Andy at Vintage Tube Services, as I feel that he is the "go to guy" for all things tubes. I usually speak to him over the phone, so I hadn't been to his website since I switched to a high speed cable modem. Loading with my previous molasses slow modem speed and ancient computer, it could take awhile to download.

Yeah, there's some good stuff there. Here's a link, so others can see why those want tubes go to Andy:
http://www.vintagetubeservices.com/

Stock List

Just in case you find navigating his website a tremendous pain in the ass, as I do. But he does have some great info!