This link will take you to a table of Remington OEM barrels. http://www.remington.com/pages/suppo...ls-tables.aspx
The "RS" indeed means rifle sight. I assume it is an 870 Express and not a Wingmaster. I only see two listed for the 870 and one has a fully rifled barrel (not yours). The other has a fixed improved cylinder choke, which is what Lee suggests.
Express Deer Barrel w/ Rifle Sights