Maybe it doesn't matter. Just be aware I haven't worked on any of the 1975 or newer versions without the distort control.
A noisy bright switch can only be one of two things. One is the switch. The switch on the earlier versions was sealed - meaning it cannot be cleaned. However, it is somewhat unlikely that a sealed switch would ever become dirty. But one possibility is that the switch is just plain worn out and needs to be replaced.
On the earlier versions, since they did have sealed switches, the most likely cause would be the pin connections. The circuit board has vertical pins, and both sides of the bright switch have "lugs" that fit over these pins. More than likely one side or the other has a lug that is loose over the pin.
The best fix is to solder the lug to the pin.