I own a Mod 66-1 with identical markings, but on both left and right sides of the gun, around the front and sides of the trigger guard.
I am the only owner of this gun. I bought it new directly from S&W (1980 I think) and it has a wider target trigger, wider than the trigger guard. I am RH, and I carried this gun in only two holsters, a Bill Jordan Border Patrol, and a Safariland 229, neither of which come in contact with this area of the gun. I have both holsters today.
My memory isn't clear about one point, but I am trying to recall. I also used a Safariland key chain. One so long, I would stuff my keys into my right rear pocket to keep them quiet, but they often came out and banged against my gun. I cannot establish that my frame marks (as stated, "like a file would leave") came from the keys but I thought it was worth a mention.
The same marks you show in your photo are on my gun, but I'm not that handy with a camera, sorry. Seems to me, the previous owner(s) of your gun tried to do a little body work, but should have left things alone.
The marks were present on the gun when I got it, just figured all these years that this was the way it came. Was no deal to me, it was going to (and did) see a lot more abuse than just that.
Hope this helps, y'all be safe.