Dan, I have a full copy of the Methuen handbook that has been scanned into pdf format. The problem with it is that each plate has some colors that are out of gamut for a RGB monitor, so it's not possible to do a true "digital version" of the plates that will accurately display all of the colors.
But for the color codes of interest, this is not an issue since the book does provide a full set of Munsell codes for each chip on the Methuen color plates. So, I can derive the conversion by starting with a Methuen code, looking up the corresponding Munsell code, then use the GretagMacbeth Munsell conversion program to obtain accurate RGB and CMYK codes (and LAB too, if that was of interest to some of the European readers).
I think it would remove any ambiguity if this full set of codes was provided for each color (Methuen - Munsell - RGB - CMYK - LAB).
Thanks for the information about Rimmel's work -- what is your opinion of the NASM data?
— Patrick Demski —