The "-AB" parts number suffix could mean any number of things, but typically denotes:
1. A design change designation. "A" is the original part, "B" is the first revision or difference, etc. These were not always issued sequentially. For example, a '73 Mercury (full-size) base wheel cover is D3MY-1130-A, but the turbine style "luxury" part number is D3MY-1130-T. There were only 3 other wheel cover/hub cap types for '73. Design changes could also be to correct a defect with an original part. An example would be making a part thicker to prevent bending or torqueing (sp?) in service in order to cure failure.
2. A color, as for interior parts. these can get quite involved due to the numerous variations but in Ford parts-speak, A is usually black, B is usually blue, D is usually red....If you were to look as the trim codes for a particular year, you could get a rough idea of how this works. I've not seen a part number suffix from this era with more than 3 characters but don't know for certain if there could be 4+.
'68 Standard hardtop, Augusta Green, mostly original, February 2015 Ride of the Month
'73 Marquis Brougham 4-door pillared hardtop, Black/Black/Black, featured in Collectible Automobile, October 2015
'02 Lincoln LS, Silver, my faithful daily driver
'14 Lincoln MKX, Plantinum Dune