The bolt-stop effect at is not a modification, but original standard. The idea was to warn the shooter in the heat of battle that his magazine was empty! The short ramp is to aid feeding. A common modification for the calmer world of target shooting was (and still is) to grind down that short ramp so that it no longer acts as a bolt stop.
Patrick
Hmmm, well then why do my other Gew 98s not have the bolt stop feature? Their followers don't have the additional vertical cut at the rear.