From :The FN 49- The Last Elegant Old-World Military Rifle.
by Wayne Johnston
On page 55
The converted rifles have "R" stampings on the bottom of the trigger
guard assembly, on the top rear of the receiver cover, on the sideof the gas
cylinder assembly, and inside the receiver in the area forward of the
locking shoulder. Some, but not all, bolt carriers have an "R" stamped
on the left, rear side. It is believed that these stampstampings indicate parts
that were modified as part of the conversion program. In additiion, some
stocks have the "r" stamp on the bottom of the forearm, justin front
of the magazine. in a limited sampl size, when the "R" cartouche is
present on the forearm, the "ARA" cartouche is absent from the left side
of the butt stock. This likely means that the "Rr" indicates stocks that
were refinished as part of the conversion program and that during this
refinish, the ARA stock cartouhe was removed. The exact meaning of
the "R" marking has not yet been determined, but there are several
Spanish language terms that would apply including "reconstrucción"
(rebuild) and "reformado" (reformed).