I onwn one and have seen several K98 bayonets with similar wear on the right bakelite grip. What piece of equipment causes this wear?
Nothing in my small collection of knives, bayonets and swords has that particular kind of wear, but it looks like it was caused by a ring. Bayonets are used for everything from cutting to chopping, digging, prying and opening cans (I'm sure that I've left a few out). With that kind of regular use, a ring could cause a distinct wear pattern consistent with the one shown.
Sounds plausible this type of wear is possible but the bayonet I have and others I've seen do not exhibit any other wear from use and have virtually all the bluing on the blade, pommel etc. . Seems they were worn but not unsheathed for any use but possibly attached to the rifle. The blades are not sharpened for hand use as a knife. It just seems the bayonets are worn in this spot on the grip but not used as a tool.