- AND THE LORD SAYETH, LET THERE BE LIGHT! -

What we are seeing here is an early PN SYSTEM that was superseded by a later system. This previous system will not show in the 1949/59 MPC TEXT FINAL ISSUE (1964). One would need a period correct CHASSIS CATALOG to ID these PN's.
BASIC PN 17503 must have been the PN for the kit and would have given a complete parts kit breakdown incl the PUBLICATION NUMBER of the sheet(s).
That's what is going on. The PN for the ACCESSORY KIT was BAD-17503-A.
Now seeing as I am too damn cheap to purchase a period CHASSIS CATALOG, these bits of wisdom flash to me (very rarely) in a typical incoherent brainstorm, usually within ten to twenty moments of consuming a TACO HELL MONTEZUMA'S REVENGE PARTY PLATTER (w/ cold drink).
That is where you will find the PUBLICATION NUMBER to help ease the search (or find an intact NOS kit).
ATTACHED - CASE IN POINT -
EARLY PN SYSTEM SERVICE PART - 17664 - PUMP ASSY.
BAD-17664-A was replaced by B8A 17664-A in the replacing PN SYSTEM.
And this current revelation answers the first post -
Does anyone know what Ford means by:
R.P.O.
BAD
BAC
BAR
I have seen, I believe these abbreviations used in the parts catalogs and the Trim and Body sealant book.
Thanks, Jim
BAD will be a PN PREFIX INDICATING MODEL YEAR the part was released for.