When I replaced the pickups in my first Yamaha SBG1000, I installed a DiMarzio SCHB humbucker at the neck, and a DiMarzio Dual Sound at the bridge. The SCHB (Super Clear Humbucker, now discontinued) came from the factory with a chrome cover engraved with the DiMarzio eclipse logo. Very cool, but the guitar just looked wrong with the exposed bobins of the bridge pickup. So I installed a plain chrome cover on the Dual Sound pickup.

Metal covers are all made of brass. There is [u]no[/i] reduction of the magnetic coupling between the strings and the pickup. Chrome or gold plating just makes the cover look diferent, again, no effect on the sound.

The cover goess over the pickup, and should fit snugly against the edges of the backing plate. Solder the cover to the plate. This can be a tricky task, because you don't want to apply too much heat to the pickup. You may want to get a tech to do this for you.