You mean the 5881's?

Hmm, could be, it's been a long time.  They may have cross-branded them for some companies.

In the late '80's and early '90's, Sovtek only offered one tube...the 5881 (5881 was a Tung Sol naming system, RCA and others called it the 6L6GB).  Its not really a 5881, its a Russian military tube for driving jet servos (yes, Soviet era jets were tube powered), but it spec'ed out the same on paper. so Mike Matthews started importing them at a time when tubes were becoming scarce. 

Some here may not have been playing long enough to know that for several years there were no new 6L6GC's and 6V6GT's being made anywhere.  And the single company that was making EL34's, Tesla, couldn't sell to the Western Wolrd because there was a trade embargo on civil war torn Yugoslavia; eventually the Tesla factory was partly destroyed by aerial bombing.  So, in the early '90's, the only options for new manufacture power tubes was the Sovtek 5881 or some woefully inept Chinese tubes.  That's why early '90's Marshalls, Fenders and other brands all came with Sovtek 5881's in them, they were the only game in town.  And the Shuguang was about the only supplier of 12AX7's a that time (once again, because Tesla wasn't an option due to embargo). 

Now, we live in a second golden age of tubes, they may not be as robust as old American and Western Euopre tubes, but we have so many great 6L6 options, EL34's options, preamp tubes and even tubes that hadn't been made in decades (KT66, KT88, 6550) or new hybrid designs.