[Arran]

The letter designates the cabinet style, in the case of the letter "X" it also used to denote the set having an inclined speaker board. However the angle of the incline became less and less as time went on till it became little more then the designation for a fully skirted cabinet. They also used teh letter "L" for lowboy, and "H" for highboy, and "B" for "Baby Grand" Philco's name for a cathedral or tombstone table cabinet.
Regards
Arran