Unless it is a commercial action it is .323. Some commercial actions were .318 into the early 20's. .318 is from the 1888 comission rifle.
The 7.92 X 57 I (not a typo) is the Infanterie round (.318) and used in the 1888 comission rifle.
The 7.92 X 57 IS is the Infanterie Spitzgeschloss or Infantry Spitzer (.323) and was officially adopted in 1905 and was in standard use throughout WWI and beyond.
The reason that you commonly see it listed a JS is that in old Euro. font, a capitol J and capitol I looked the same. This is the same reason that there is no J street in Washington D.C. (see snopes for this one).