I must admit that I had not noticed it, and cannot recall why it is there. Probably not deliberate. The only time I would scribble on a sole is when lapping it, and this #103 did not need lapping. My first thought was that they were scratches, which is possible, since the plane has a long history of use (the first new plane I ever purchased, about 20 years ago).
Regards from Perth
Derek
Or, if what you need is a plane for coarse work, you could get, say, a Stanley 220.