Are you sure the buffer is a factory part? Darned few guns have them in their stock, straight-from-the factory form.
The only H&K Manual I could find for a P9S was in 9mm, so the parts lists and diagrams weren't any help.
I suspect the buffer was an addition by a prior owner. If so, your weapon will probably work just fine without and arguably be more reliable, too.
Just getting an owner's manual for that specific gun/caliber may answer the question as to whether it's even needed...