Mesa had some 5881s that they were labeling 6L6s years ago. In fact on their data comaparison sheet they even noted that they were in fact 5881s though marked 6L6s.
The Sovtek 6L6GB is most likely what is now branded as the 5881WXT. Sovtek also manufactures a wafer-based 5881 that is otherwise identical to the full based 6L6GB/5881WXT. The wafer based version is rebranded by Mesa as their STR-425.
