I've got a buddy with a 1967/68 twin reverb that has a black faceplate which looks original, ab763 circuit but a pt and ot with 1968 date codes. He says the amp is original but I've never seen 68 codes in a blackface. I know tube charts are deceiving but was there some overlap in 68 where they used up the last of the black faceplates? Or is it more likely someone changed the plate before my buddy got the amp? I'm only concerned because it may become my amp soon. Thanks.