Bill, I do not understand your use of the term "paint code" per se
The 1955-2nd, 1956, and 1957 Task Force Truck Data Books all provide RPO 234 letter codes for all exterior colors, and all three TDBs provide paint codes (Dupont Dulux paint codes and Combination Numbers). I do not see trim codes in the TDBs or in the Specifications Manuals for any of those years.
Do you see trim codes in any of those years' Specifications Manuals (on what pages)?
It looks to me that Dupont used the following Combination Numbers on paint COLOR BULLETINS:
500s as Comb. Nos. in 1954 (BULLETIN 26)
600s as Comb. Nos. for 1955-1st (BULLETIN 27)
????? 1955-2nd (I'll look for a paint sheet) - probably the same as 1956
700s as Comb. Nos. for 1956 (BULLETIN 28A), 1957 (BULLETIN 29), and 1958 (BULLETIN 30)
900s as Comb. Nos. for 1959 (BULLETIN 31)
