"I am working with a early 1900's vintage SxS
Box Lock"
The meaning of a 'R' AFTER the serial number is uncertain but may indicate the A.P. Curtis Forend Fastener used 1912 to 1920
Regular frame automatic ejector ('RE' BEFORE the serial number) on left with Specialty grade 'S'. Featherweight frame 'Fw' and 'Field' grade on right. Featherweight guns 1909-1913 may not have the FW prefix stamp.
Look around here a bit Dr Brown
http://www.picturetrail.com/sfx/album/view/15234180 and
http://www.lcsmith.org/index.html