1915 German barrel drawing indicating shoulder angle of 18° 26' 5.8''.
Source: Rifle & Carbine 98, M98 Firearms fo the German Army from 1898 to 1918, Dieter Storz, 2006, page 109.
1932 German (Mauser, Oberndorf) maximum cartridge and minimum chamber drawings indicating shoulder cone angle of 38° 15', which equals a shoulder angle of 19° 7.5' (1/2 cone angle). It appears the shoulder cone angle is rounded up to the nearest 5' of angle. The actual shoulder angle is 19° 6' 2.8'' per the linear dimensions.
Source: Mauser Original Oberndorf Sporting Rifles, Jon Speed, et al, 1997, page 276.
1938 SAAMI maximum cartridge/minimum chamber drawing with revisions up to 1947 indicating shoulder angle 20° 48' 25'' from 1938 to 1947. SAAMI changed drawing shoulder angle to 19° in 1980 per Forster Products, Inc.
1967 Yugoslav sporting rifle standard (CIP) 8x57mm JS chamber drawing indicating shoulder cone angle of 38° 15', which equals a shoulder angle of 19° 7.5' (1/2 cone angle). It appears the shoulder cone angle is rounded up to the nearest 5' of angle. The actual shoulder angle is 19° 6' 2.8'' per the linear dimensions.