My understanding the + was a modification to the existing or a new cast mold to denote a cut off and change point in grooves.
Talking with a guy during a round that had a beautiful full set of BeCu irons from 1 to wedges that were not the + models sent his back for legal re-grooving. They did not have a plus marked into the head but did receive a certificate of conformance from Ping which he carries a copy of in his Golf bag.