The alphabet soup of prefixes and suffixes you find in current manufacture tubes are mostly for marketing hype purposes, and have little bearing on what it all meant back in the old days.
All this applies to the large octal base tubes. When 6L6s and 6V6s first came out, they were in a metal envelope. When the first glass envelope versions were introduced, they had a G suffix applied (6V6G, 6L6G). These first glass versions used the large bulbous shouldered envelopes. Then they went to the smaller tubular straight sided glass bulb, a T suffix was added, hence the 6V6GT (glass, tubular). A later beefed up version came out, which was called the A revision, hence the 6V6GTA
With the 6L6, they additionally did some internal mods to the guts to increase it's power handling, so the newly revised design became the A revision, so we had the 6L6GA. More revisions and beef-ups followed, giving us the 6L6GB and ultimately the 6L6GC.
STR, depending on who you talk to, stands for "special tube request" or "special test request". Basically what this meant back in the day, is a large OEM customer would request the tube factory to test or tweak an current production tube for a specific operating condition or parameter. Like operating base-up in a high vibration environment, for example. Today, it's just more Mesa BS marketing. Mesa does not make tubes, they just buy them from the usual suspects and put their own labels and nomenclature on them. Mesa resells a couple three different versions of the current production 6L6, so they need a way to differentiate them from one another.