I have re-read the section on duplex metering but I'm still not sure if they consider an invercone to be equivalent to a flat receptor or a dome.
And now I have read the section specific to invercones and understand it a bit better. The book says that the invercones on the older meters such as the II and III are to be considered the same as a flat receptor and would benefit from the duplex method of averaging between the pointing at the camera and pointing at the light source readings whereas the newer IV and V meters had invercones which compensated for this and only needed the single reading taken whilst pointing at the camera.
"People who say things won't work are a dime a dozen. People who figure out how to make things work are worth a fortune" - Dave Rat.