Every artillery I have ever seen or owned had the fine tune sight....and then I get this one. Low serial number 1917, appears all correct. although the lug has been ground
What does the other side of the pin holding the sight on look like? The side visible in your photo looks almost like it was a peened-on addition, and I think the germans would have used a pin with a cupped end so it could be pressed in.
Which is a cheap and easy way to color the metal. I fire blued all sorts of steel pieces with the alcohol lamp from my Gilbert chemistry set when I was a kid. Lamping, they used to call it.