A standard UKmarking - BUT - brand new butts have been seen (primarily in the USA
) with the FR marking and unsubstantiated supposition is that it was either a 'contract' marking or a 'supplier / manufacturers' mark.
From :
R.E.M.E. CENTRAL INSPECTORATE
INSPECTION STANDARDS PART 3
(PROVISIONAL)
"APPLICATION OF GAUGES AND EXAMINATION OF SMALL ARMS AND MACHINE GUNS"