German sources from 1918 are scarce, most are from a later time.
A Rheinmetall drawing from 1934 identifies the caliber as 13 mm and case length as 92.0 mm. Therefore, a metric designation of 13x92R seems correct.
SR means semi-rimmed, but the case is of the ordinary rimmed type. I have to admit that Wolfgang Kern in his book used 13x92HR (HR = Halbrand = semi-rimmed). Manfred Stegmülller in the ammunition chapter of Kern’s book in my view correctly writes 13x92R. Stegmüller also writes that no tracers were found so far.
Nominal dimensions of the bullet according to the Rheinmetall drawing: diameter 13.2 mm, length 63.4 mm, weight 52.5 g.
The German official designation, according to a label on a casing, was “Tuf-Patrone” (as usual no caliber in the small arms cartridge designation). Tuf means Tank und Flieger (in the sense of anti-tank and anti-aircraft). The cartridge was intended for a Maxim type machine gun (Tuf-MG) and the Mauser rifle only a stopgap measure.