I have a copy of that book as well, Rick, It has some good/great information but very hard to FIND in the book as the organization is just a 'collection' of GM paperwork. That book was 'compiled' by two old members of the NNC who had access (via the NNC) to a lot of GM documentation.
I was referring to the same codes I think as you were, and my source was Pat Chappel's 'The Hot One' (which also had access to the same documentation as the two NNC fellas who wrote the book you referenced. Pat Chappel was (maybe is still?) a member of the NNC as well when she wrote the book!
Here are some pages which I scanned from the book for the 1955 Trim and Paint Combos in the back of her book; I suspect the same pages are also in the '55-56 Restoration Guide'.
1955 Trim combos.jpg
1955 Paint Combos.jpg
1955 2-tone paint.jpg
I've mostly studied this information from my 'Nomad interest', and for Nomads it's very simple, but for the more common models (under models and model usage in the charts), it can be very confusing and I admit I've never totally understood what all the suffix codes mean. If anyone can explain those .. I'd love to hear it!