So what do you think? man-made or machine? Counting the days, or just careless contractors?
There are 2 bricks like this at the back of the fireplace, I never noticed them before. I guess just one little ray of sunshine must have gotten right to the back, and they became clear.
The scratches appear quite clean, suggesting that they were made after the brick became sooty, i.e. quite recently. What is the structure above in the photo - is it a register plate?
Can't imagine what machine could have made those marks so regularly spaced, so I'm imagining accidental, like something serrated leaned up against the brick which got shunted. Just another little mystery in the house's history...
Has someone trimmed brickwork above to get a register plate in?
If so, it might be someone being a bit careless with a scutch comb - although they are usually no more than about 2" wide, at least the ones I've seen anyway.