If it's a late 70s SG-1500, it will have a maple top. It should look similar to the guitar in Ratrod's second picture above, but with chrome hardware and dot markers (I think!). Does that sound right?
Covers are supposed to affect the tone slightly, they reduce the highs and warm up the bass. BKP can compensate for this effect in the winding, but I'm not sure if that's done as standard or if you have to ask :? . In the case of the Mules, I think Tim originally "voiced" them with covers.
All my BK pickups (so far) are covered and they certainly don't lack brightness - I wouldn't worry about covered vs. uncovered except in terms of looks.