There have been many instances of wrong stamping on the compliance plates. On the other hand, are you really sure that there were no cloth inserts on your car to begin with?
My car has the opposite problem, ie only a "b" on the plate but i know for sure that the car had cloth inserts from new (provinance).
Looking carefully at the 72 capri brochure the standard features for XL and GT included "basket weave" inserts and under optional extras is the "cloth inserts". Although the brochure doesnt us these terms and as a result seems ambiguous it can be understood by including the illustrations.