After July 7th 1896,
Barrels made after July, 7 1896 = ((0.15mm)+(7.9mm)+(0.15mm)) = 8.2mm = 0.3228346” or ~ 0.323”
MARKINGS, on 1888 rifles
= 2mm diameter dot = Modified barrel contours, post 1891.
Z = 3mm high = Deepened rifling groves, post 1896. A large 7mm “Z” may also be stamped on the right side of the butt.
S = 3mm high = Rifles suitable altered for S-Munition. 1903-5. An additional 7mm crowned “S” may be found on the right side of the butt.
n = 2mm high = Signifies an 88/14 conversion, 1914-15.
The preceding information was from “German Military Rifles and Machine Pistols 1871 – 1945”,
By Hans Dieter Gotz. Translated by Dr. Edward Force, CCSU. Includes information on assault rifles and Sten copies.
As I understand it, the adoption of "s" munitions required an opening of the chamber throat for the larger bullet.