The world of NOS tubes is interesting, indeed. Please note that "NOS" means "New, Old Stock." Unfortunately, it is common to see so-called "NOS" tubes that are very clearly old, but not new. Of course, I don’t know whether that was the case in your instance.
... two pairs of Mullard EL34s, one pair labeled Amperex and a second pair came in Mullard boxes ...That sounds odd.