Our house was "modernised" in the 1970's.
I don't know if this was ever a general system, or bricolage by the previous owner, but the curtain rod in one room was supported by two solid square section steel rods, about 1/4" diameter and 6" long, with a point at one end, driven into the brick wall.
There were screws about 1/2" long tapped vertically into the other ends, which I assumed were originally designed to hold something fancier than the round wooden rod which was sitting loose on the supports.
I removed the supports when I insulated the inside of the wall, by hammering them gently from side to side until they were loosened.
EDIT: I just found a picture of something similar
Atelier de forge ROBERT & ROBERT : Tringles anciennes
Scroll down and click on the image of the yellow curtain. Maybe the original bar was missing from ours.