The bolting on Remington Hammerless Double Barrel Shotguns started out as Purdey double underbolts with a third fastener, in the Remington case the rib extension, middle barrels in the picture --
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.
