the "s' simply stands for spitzer , the bullet dia would be circa .322---.323..
the reason i say i don't shoot 8js ammo in my gews is 98's have a third locking lug on the bolt the gew88 doesn't and the 98's also have venting holes in the bolt to ensure the shooter isn't hurt from gases from a case rupture,the 88 bolts are not as sophisticated, there is also the split bridge affair on the gew .
when it all boils down to it in theory you can shoot .323 in your 88 ,how does that saying go?,,,,, "if it chambers easilly enough it's good to go" or something like that ,anyway i choose not to.
if you pull a bullet from a commercial say win / rem 8mm and mic it you will find it measures closer to .321, for fear by makers of people shooting their ammo in the 88's (it's a lawer thing!).
all my 88 have no "s" on the reciever so they are not reamed to accept the .323( although they probably will ).
the circle z was a czech stamp and most of the rifles with this stamp were sent to south america, and the bore dia on these rifles can run as tight as .316.
when the turks got hold of your 88 they re-chambered it for the spitzer and it probably shot .323 all it.s service life.
me i'm a wuss so i cast up to a .321 bullet and load light at around 1200-1400 fps ( no need for a g/c at these vel either).
sorry for wandering a bit but i tend to do that alot.