Matt Lukin Precision Bass

I've been getting into a lot of grungy stuff lately, specifically bands like Melvins and Mudhoney, and I noticed that in pictures, bassist of both bands, Matt Lukin, can be seen playing a Fender P-Bass with an unusual metal control plate on the pickguard. Or maybe it isn't, I dunno. Can anyone give some insight on to what it might be?

I also see a small knob or button above it, if anyone has any ideas I'm all ears.