Hi Dent42,
thank you for you kind words...
I will try to explain how I did it (sorry if it is a long explanation)
The truth is I was going to stop the serrations before the "little ski jump" (don't know the technical name).
I wanted to finish in a straight line where the red mark is. I used a zip tie to limit the file strokes and end there in a perfect straight line. However, the zip tie that I used was too thin and/or I pushed the file too hard, broke the zip tie and marked the ramp all the way to the flat top of the sight area. I didn't do it once but several times.
I though of erasing the marks, but I was concerned of creating a "curve" or a "dent" on that area that would look odd (and damage "the ski jump").
Since the marks where really thin, looked like "hair lines",I decided to make them bigger with a needle file I bought at Brownells some time ago (and that I almost never use).
I did this slowly and carefully, taking my time.
Then, I used the piece of checkering file that I cut, to deepen the lines and try to blend them with the others. Despite being rectangular, the file "cuts" at an angle. Since the lines where already made "thicker", that served the file as a "channel" or "guide" to keep the straight line...
I left several marks that I later cleaned with some stones and sand paper... (had to repeat the process a few times).
I am sorry I don't have better pictures of the process. I thought of recreating some of the pictures with the actual slide, but it is already blasted and cleaned, just waiting for some good weather, so I can do the hot bluing...
At the end, this was a mistake that luckily ended looking ok. However, I think that if I was going to do it again, I think I would finish in a straight line just before the "jump"...
hope this helps...