For those modeling from the 80s on, there is a different set of AAR car type designators…
In the mid-1970s, the AAR created it’s UMLER (Universal Machine Language Equipment Register) computer database. In the process, the did away the letter based car types, most modelers use, and replaced it with one using letters and numbers.
The new system used a letter to denote the basic car type, and then a three digit number to identify the specific car class. Covered hoppers, intermodal, and auto racks received their own identifiers, instead of being L type cars, and passenger cars lost their identifiers, and were lumped in with the company service cars.
Basic Car Type Identifiers:
A – Boxcar, equipped (plug door, load restraints, etc)
Alpha-numeric code giving a general physical description of the type of car. (Refer to Section VI, Exhibit D, of the UMLER Specification Manual.) Refer to Appendix for additional information.
AAR Mechanical Designation (4 character alpha)
Mechanical designation is dependent on AAR car type. (Refer to Section V, Item F of the UMLER Specification Manual.)
I did find a downloadable copy of the UMLER Specification Manual on the railinc.com website (via google) - 3MB PDF - good reading.