The 318 bore is actually pretty rare to
see now. But if there is any question,
have a gunsmith "Slug" the bore ang make
sure. The 1888 Commission rifles had a 318
bore, I have seen 1 1891 Mauser in 318 bore
(VERY RARE) and the early 98's had 318 bores.
As I understand it, Anything made after the second year
of WW 1 should be .323 bore. (I think it was earlier though).
The " I " (INFANTRY) designation was misread by by some government translator as a " J " after WW 1 and it stuck.
The " S " (Spitzer or POINTED) designation was given
to the pointed .323 bore bullets when they were adopted.
If I remember correctly, when a rifle was converted from
.318 bore to .323, a large S was stamped on the Receiver.
** Just a note for confusions sake though **
I recently aquired a "batch" of 200+ Grain Cupro-nickel
jacketed roundnose 8x57 ammo dated 1926 on the headstamp
that was .318 diameter !. Can't Figure that one out.
"WHEN IN DOUBT,,,MIKE IT OUT"
Hope this helps.