I agree the mask is not the norm but they are easily switched/changed etc. and Bradshaw did wear several different style masks thoroughout his career. (He wore two similar Dunguard masks, the DG 105 and the DG 205, in the early 70's and two nearly identical but slightly different Schutt OPO, green dot masks in the late 70's.)
What is more troubling to me, (in addition to the previously mentioned date code and label markings), is the fact that so many Steelers wore MaxPro's in that era. Which, in my mind, creates lots of room for reappropriation of a player's identity. This is especially questionable, IMO, for a fund raisng auction where provinance, authenticity, etc. could be minimized.
Reminds me of the Johnny Unitas helmet that sold at auction for a large amount of money a few years ago. If I remember correctly it was given to a young kid who was a family friend of the equipment manager of the Colts. The young kid, now an adult, claimed the EQ manager told him that is was Johnny's game helmet. Only one problem, it was a completely different brand and model (MacGregor or Rawlings I believe) from what Johnny wore (a Riddell) throughout his career.
In any case, a Steeler's clearshell MaxPro is highly coveted and collectable (with or without authentication of player usage) and therefore a very valuable piece, IMO.