My recollection from a previous article was that in many cases, ovens were communal, so the mark was so that they knew which items belonged to a particular baker:
I guess the share your home BS started hundreds of years before my time makes life seem more futyl
That is correct. In my home country, Spain, such ovens still exist, although nowadays they are few and far between. The few people who still use them keep signing their bread, a tradition that goes back to at least the Ancient Egypt. You can read a bit more in this Wikipedia article (no English version, sorry)
https://es.wikipedia.org/wiki/Sello_de_pan