The Invercone for Master II and III is different, Ian. The one you linked instructions for is the improved version that read correctly for both metering ranges.

I have an invercone for my Weston IIIs (just the white part) and from what I have read BrianL is right -- the "ND" part is a strainer, like the flap, not a real ND filter. But since those seem to more-often-than-not be lost I plan on figuring out what ND filter to replace it with.