I think the 6l6 is a regulator. Gibson did this sort of thing on a few amps. The GA86 used a 6V6 and the GA100 used an OC2. Your amp doesn't appear to have the right tube compliment for either. But they also made impromptu circuit changes, sometimes dramatic and amps that differ from any known schematic are common. They also recycled names a lot. I don't even know how many different "Les Paul" amps they made. Run a *oogle search with something like: vintage gibson ga "regulator tube" :and see what that turns up.