One document I saw (http://www.nakina.net/other/aartype.html) derives its list of AAR Mechanical Designations from Official Railway Equipment Registers starting in 1917. The range of years to which the designations apply also are listed. 

"XA" -- Automobile. Similar in design to general service box car, but with exceptionally large side or side and end doors (1/17-7/32).

"XA" -- Automobile. Similar in design to general service box car, having 10 ft. or greater clear height inside but with side doors at least 10 ft. wide, with or without end doors (4/38-7/43).

"XAF" -- Automobile-Furniture. Similar in design to automobile car, but usually of larger cubic capacity and greater inside clear height (7/32-10/66).

"XAR" -- Automobile. Similar in design to general service box car, but with side doors at least 10 ft. wide, with or without end doors, and equipped with permanent automobile stowing equipment (4/38-1/49).

"XAR" -- Automobile device. A house car similar in design to "XM" box except unlined with side or side and end doors and equipped with loading racks and/or floor tubes with tie-down chains for loading setup automobiles and trucks, not suitable for general service loading of all miscellaneous commodities (1/53-10/66).
It is recommended that in each case where loading racks in "XAR" and "XMR" cars have been made inoperative by securing in roof of cars for the purpose of transferring cars to general service loading, a note to be included in the Official Railway Equipment Register showing series or individual numbers of cars affected.

"XF" -- Furniture. Similar in design to general service box car, but usually with greater cubic capacity (1/17-10/66).

"XMR" -- Automobile Device. A house car similar in design to "XM", fully lined with side or side and end doors, equipped with loading racks and/or floor tubes with tie-down chains for loading setup automobiles and trucks and suitable for general service loading of other miscellaneous commodities (1/49-10/66).
It is recommended that in each case where loading racks in "XAR" and "XMR" cars have been made inoperative by securing in roof of cars for the purpose of transferring cars to general service loading, a note be included in the Official Railway Equipment Register showing series or individual numbers of cars affected.

"XR" -- Auto Device Car. A house car similar in design to "XM" box either lined or unlined, with side or side and end doors and equipped with loading racks and/or floor tubes with tie-down chains for loading setup automobiles and trucks (1/68).
It is recommended that when loading racks in "XR" cars have been secured in roof of cars for purpose of transferring cars to general service loading, a note be included in the Official Railway Equipment Register showing series or individual numbers of cars affected and whether such cars are lined or unlined (1/68).