Interesting, thanks, Valdas! The Technical Information datasheet that Guy sent me specifically notes ISO 320:
Exposure index: ISO 320 / 26 - The speed value refers to a developed N75 average gradient of 0.65. The exact exposure index depends on the prevailing shooting and processing conditions. Special feature of light meters, shooting conditions, planned artistic effects and deliberate processing changes are to be taken into account in setting the lens aperture. Rehearsal screen tests are recommended.
I guess it's time for rehearsal screen tests by our forum members here! Or maybe they just took the average of the daylight and tungsten ratings.