Stop, before you start machining a new Tangent rear sight base, have a close look at the dovetail in the rib if it was really set up originally for such a sight. These tangent sights were fitted in dovetail cuts about 2" long on ribbed barrels. Tangent sights were only one option on Mauser B rifles, more often they are found with more conventional and practical hunting type sights, most often consisting of a standart with one or two folding leaves, see Jon Speed's "Mauser-Original Oberndorf Sporting rifles" page 135.