It would seem that pentax changed the hood for this lens. According to my official leaflet, the correct hood for the Pentax 645 A 2.8/45mm lens is the RH-RA 67mm screw-in rubber hood, although this contradicts the info above. This information is based on a Pentax 645 Interchangeable Lenses folder dated November 1995, just before the release of the 645N; see the attached picture (sorry for its poor quality).