Post-1913 guns usually had "Field" or a letter beneath the SN indicating the grade ("I" = Ideal)
The meaning of a 'R' AFTER the serial number is uncertain but may indicate the A.P. Curtis (Pushrod) Forend Fastener used 1912 to 1920. Dr. Stubbendieck believes that it may mean that the shotgun has extractors rather than ejectors.
Please be aware that the gun originally had 2 1/2" chambers. There is more information under the FAQs.