I'm not sure if the marking has something to do with the FTR process or not. However they only seem to appear on the barrels and in my case all the rifles received a new barrel in 1953. On any of my no4's from the forties or pre ww2 no1's they are not present. I also don't recall seeing any mention of them in any of the enfield literature that I have. Trying to decipher the hundreds of proofs and marks on enfields is part of the fun of collecting them!