Nearly all the pre-1912 commercial Mauser Bs have the military style floorplate. If the tangent sight on your rifle were still present, you would see the purpose of that tiny flat on the barrel reinforce: It allowes the tangent sight to be set in it's lowest, 100m position.
Good luck in finding an original, pre-1912 stock! Tese came with a round pistol grip, tapering, no Schnabel, foreend and half-oval, C-shaped side panels.