Not so. There is nothing peculiar about your rifle - there were 2 major stages of development of the Gew.88:
1) The "S" (spitzer) bullet was approved in 1903. Pre-1903 Gew.88 barrels were reamed out in the chamber to accommodate the slightly fatter neck of the "Patrone S"cartridge with the 0.323" bullet and provide a longer lead. Rifles built or adapted to take the "Patrone S" were marked with the S stamp.
2) The Gew.88/05 was approved in 1905. This version had the modified magazine and loading bridge for the Gew.98 -type loading clip.
As usual in such matters, production limitations meant that by no means all rifles made it back to an arsenal for upgrading. So there are Gew.88s around in various stages of upgrading - original, S-barrel, and '05 conversions. A hand-me-down policy of distributing new rifles to first-line troops and passing older versions on to reserve units and the colonies meant that the older the rifle, the more likely it was to end up in a rack in the backwoods and remain undisturbed for decades!