The bright switch on the models is for a bright cap, a capacitor across the volume pot on many amps which lets some of the high end through no matter the volume setting. Amps that had it on a switch could connect or disconnect it to get either a more even sound at all volume/gain levels, or keep it connected to make for a brighter sound with more high end at lower volume/gain settings which would even out as the knob was increased. And on amps where it wasn't switchable, it was a very common mod to either add or remove the cap, or adjust it's value for a different amount of bright frequencies.
(I say volume/gain because on a clean channel it would be called volume more often, but on an overdrive channel it would be called gain. but in both cases, it was just the first input gain control.)
The 5150 II/6505+ amps also had a bright switch, but it wasn't actually a bright cap. It was a separate tone network that would engage a treble cut if bright was disengaged. So although the models had a bright switch, this was the bright cap that is available on every amp model on the ideal controls, and not the bright function that existed on the amp. So now we can mix and match both.
Sorry if it's been asked before!
