Proof rules of 1968, but there was no complete changeover. The "Jahreszeichen"=year date has to be applied with the proof marks (eagle/N fi)and the proofhouse stamp. "The Jahreszeichen consists of the last two digits of the year, to which the digits of the month may be added. On applicaton by the maker the digits 0 to 9 may be substituted by the letters A to K."
Obviously this is to camouflage the fact that a "new" gun may be standing on a dealer's shelf for quite some time.