Legacy Question from a new kid on the block

[marlingschap]

Hi Folks, I have a legacy made for Jerry Donahue in 2002. The middle knob has been modded with a grubscrew which makes it a two position switch. Any ideas what this might do? Is it like a phase switch? leads go to all three pickups - I looked inder the pickguard.

Also, the bridge pick-up (only) has a thick copper plate screwed to its base - is that for any reason.

One final question, when the guitar is on and volumes up, theres a very faint buzz or crackle when touching the strings.

I promise I won't ask anu more questions until i've had some replies. Pics available of the guitar if needed.

Cheers

Rickeroo