What is the purpose of the 2 additional tapped holes in my jaws (not the holes for mounting on the chuck)?

The 2 "supplement holes" on those 2 jaws are originally designed to allow users to attach their own set of wooden attachments/ jigs on the chuck.

This design dates back a couple of decades and since then many other type of jaws have been developed, pretty much eliminating the need of those holes. Although for most people this hole may be unnecessary, some people out on the market may still use it to put some of their own attachments on the jaws.