Previously I would have guessed the 70 , but having read the words of the designer, Jun designed this lens at 77mm because 77 is a lucky number in Japan. He also said, although I can't be sure the translation was exact , that this series of lenses was not designed to do well in MTF scales.
The abstract from Jun Hirakawa's technical report...
Quote: Abstract;This paper introduces features the titled lenses on the following items: 1) Standard and middle range telephoto lenses for a 35 mm single-lens reflex cameras sold by Pentax, 2) design concept and performance; unique expression with high quality picture, 3) exterior; machined aluminum body to provide high grade sense, 4) focal length; 43 mm (diagonal length of 35 mm size film) and 77 mm (lucky number among 70-80 mm), respectively, 5) both AF and MF operation possible, 6) helicoid screw at focus part provides torque feeling, 7) realization of smooth AF operation by a little reduced gear ratio of drive unit and 8) lens aberration compensation; priority to practical photographs expression than numerical evaluation.
In other words Hirakawa didn't give a crap about MTF... he just wanted good pictures.