Hi Slug, here's something from
http://www.9mmlargo.com/enfaq.pdf (perfect first read for me when I just got bit by the Enfield Bug):
"Three firms in England made the No4 rifles. Each firm was assigned blocks
of serial numbers so that no two rifles would have the same number. The
number is stamped (or sometimes etched!) on the left side of the butt
socket.
Manufacturers markings are as follows:
FY or ROF(F) = Royal Ordnance Factory in Fazakerly, Lancashire
M or RM or ROF(M) = Royal Orndance Factory at maltby, Yorkshire
B or 85B or M 47 = BSA-controlled company in Shirley, near Birmingham
The word ENGLAND is often stamped on the receiver ring of these rifles.
No5 Jungle carbines were manufactured at Fazakerly exclusively. Beware of
fakes!
Additionally, No4 MkI* rifles were made in Canada and the US. The
Canadian rifles, made at Long Branch Arsenal near Toronto, Ontario, were
marked LONGBRANCH on the left side of the receiver. Rifles made in the
US by Savage Arms Co., (in the former J. Stevens Arms Co., plant in
Chicopee Falls, MA) were stamped U.S. PROPERTY on the left side of the
receiver, to satisfy bureaucratic necessity under the Lend-Lease
arrangement with England).
Sometimes a No4 will be found with overstamped markings, including the
cryptic letters "FTR". This indicates the rifle has undergone a "Factory
Through Repair", a complete Arsenal overhaul and reconditioning. Most
"FTR"s were done after World War Two, and the rifles will be stamped (or
over-stamped) with post-war dates. All such work was done at Fazakerly
Arsenal."