The picture at left shows a Russian 6L6GC rebranded as "7581A, W. Germany" by International Components Corporation to the left of a real General Electric 7581A. The second picture shows the same fake 7581A next to a correctly branded Russian 6L6GC. Note the taller bulb of the Russian tube and the fact that the black base is slightly larger than the glass. Also, as far as I know, all real 7581A tubes, no matter the brand, were made by General Electric. All should have the type number acid-etched on the glass.
The 7581A and 6L6GC will test the same on tube testers. however, the 7581A has a 5 watt greater maximum plate rating (35W vs 30W)
Compare these and the other 6L6GCs listed in the Hall of Shame.