Please ignore Bob Q's recommendations. He thinks he knows everything on these and yet his posts indicate he knows little and his advice is just plain dangerous. Turk 88's are well known for having all sorts of different bore diameters as Eaglelord has pointed out. Specifically the notch was made to allow the spitzer bullets to be loaded from a charging clip. While the bullets themselves were not longer, the entire system was. Loaded individually, they would still fit without the groove but the charger put them in at a slight angle. This is part of the 05 upgrade program and specifically for the spitzer bullets.
This comes up every time someone asks about the 88. There is an excellent reference book on these rifles by Paul Scarlatta that I and I believe Eaglelord have and reference. Bob thinks the guy is an idiot and wrong about everything. Your choice on who to believe but I for one go with Scarlatta, GermanGew 88 Commission Rifle, A Collector's Guide. Isn't expensive, it's paperback, $30.