Zimhunter
In the old German writing style the only difference between the capital J and I was a small line to the left on top. So the J without the small top line to the left was a I. I stand for infantry with introduction of the lighter “Spitzer” bullet the change was made to 323 bullet diameter and the S was added for Spitzer from there IS Infantry Spitzer indicates the change to 323.
However commercial manufacturing for civilian rifles carried on and only gradually the 323 version got introduced to the civilian market you find 318 manufactured rifles and combination guns build up to WW2. I don’t know of any new build German rifles in 318 caliber after 1948.

Bonanza

This is a strongly build quality gun and could be actually been build by Merkel for the trade sure looks like it. Will last yours and your kids lifetime the caliber is underrated and will kill anything very efficient in NA and most of the stuff in Africa as well!
Bullets in 318 are made by Norma, Woodlight, RWS, GS, Rhino etc. get a good supply of 196grs and you set for life, as cases you can use 8x57IRS no problem. Reloading dies 8mm mauser for the rimless will work fine but you have to get 318 expander for it or just sand the 323 expander down get a Lee factory crimp die and you set.

If there is no “S” marking on the barrel it is most likely a I 318 bore just slug the barrel or make a cast to be sure.

German manufactured ammo in 323 have a black ring on the primer an extra grove on the visible part of the bullet and extra marking on the box.