You don't mention where this number appears, most probably a screw up on the
line or some other number overstamped. check the door tag, title and both
fender stampings to see if they all have the "A" on the end .Has the Door
tag been replaced? how about in the cars history has the inner fender been
replaced? Some times in rust or crash repair a shop may restamp the number
incorrectly. Does the vin stamping on the fender have the little star marks
at the front and rear of the stamping? The 5th digit "A" is for the engine
code. in 25 years in the Mustang bizz  I've seen overstamps, miss stamp, and
missing digits often. However, not in all places is the screw up repeated.
Except on several of the 70 boss cars the 5th digit was omitted in all
locations except the windshield tag.
hope this helps