Are these vintage Sylvania 6L6GC tubes?

Can't tell without seeing them. 
You can put photos up on photobucket. Com and link here. 6L6 are not very scarce (except for NOS RCA black plates) so you won't be retiring anytime soon. 

There was a special Sylvania 6l6GC version spec’d by Fender many years ago.

Later on (early 80’s?) Mesa Boogie spec’d their own 6l6GC, but I’m not certain who manufactured it.

Maybe try a guitar site/forum?

I checked the Tubemonger tube library (great pics of tubes) but didn’t turn up much on the 6l6GC.

DeKay