Crossed swords with letters is a Birmingham proof mark, letters are a date code. Code started in 1950 with letter A, so letter O would designate 1965 as the year it was proofed for the civvy market.
Are you sure that it is an R and not a P? P in box mark would suggest that it has at sometime in its service life been through depot repair at the Poole facility. There are Enfield inspector marks on the right hand nocks form of the barrel which tells me it has had barrel work.
There is a date mark on left barrel nocks form '17 this designates the year of the Govt acceptance stamp. There will be a broad arrow mark close by. So barrel work was done in 1917.
Not unusual for a 1916 rifle with wartime use to be damaged on the battlefield and returned to a facility for a new barrel within the year.
Some markings have been obliterated by the civvy proof marks .BNP 303, 2.222, 18.5 tons per square o, The area has been buffed and some martial markings removed before the civvy markings were applied.
The rifle was in and out of the armourer's hands over a projected period. The markings on the left butt socket tell me that it has been govt inspected, in 1925, 1934 and 1959. Which indicates that this puppy had a long service life.
