According to my Pentax 645 SMC lens manual, the correct hood for the 45/2.8 is the RH-B70mm, which is described as a "slide over" type of hood rather than a screw-in. That explains the 70mm diameter. It is the only slide over type of hood mentioned in the hood chart of my manual. The others are the RH-A58mm (75/2.8, LS75/2.8, LS135/4) RH-B58mm (55/2.8), MH-RA67mm (120/4 macro), RH-A77mm (80-160/4.5), and RH-RB77mm (35/3.5) Of course, the 150/3.5, 200/4, 300/4 and 600/5.6 all have built in hoods. This manual pre-dates the WA zoom so it is not mentioned.