These early guns were named "bridge frame" guns by Charles Semmer who wrote the book on Remington Doubles --
A few years into production they simplified their frame milling by eliminating the "bridge" and used two small tabs on the foreward part of the bolt engaging notches on the sides of the barrel lug, rear barrels in the picture above --
By very late Model 1900 production they eliminated the foreward tabs on the bolt and the milling of the barrel lug and just used a single underbolt and the rib extension, front barrels in the top picture.