You've nearly worked it out for yourself! The incident light attachment (Invercone) will not fit on the meter with the high-range baffle closed, so for a high-range incident reading, you open the baffle, insert the ND filter (which will not stay in place by itself) and fit the Invercone on top. For a low-range incident reading, it's baffle open and Invercone alone with no ND filter. From the Master IV onwards, Weston changed to the more logical and simpler solution of an Invercone that fits either with or without the baffle in place.
Originally Posted by Steve Smith