I don't know about that particular type of rifle, but often letters are used to designate variants of a model. For instance, on Remington 870 shotguns, an M at the end of the serial number indicates that it's a magnum receiver, and can handle 2 3/4" or 3" shells, where as (IIRC) an SM indicates a super magnum that can also handle 3 1/2" shells.
Also, according to my very quick research, no more than 285,000 or so of those rifles were ever made, so I doubt that they ran out of numbers.
Edit: After a little more research, I found this page: http://www.wisnersinc.com/additional...erl_88_100.htm
It appears that the A may indicate a replaced or duplicated receiver.