OK, I was thinking of the M1907 Dreyse, but now I am at least on the right track. But the problem is that I don't have one of those, and can't locate a diagram. I will suggest a couple of possibilities. One is that the indicator pin may be made as part of the firing pin or as a permanent assembly with the firing pin. With guns like that, the best way of making a pin is to simply turn one with two diameters, like a nail*. The larger end will be of a size to fit into the firing pin and be held in by the FP spring. The main shaft is of a size to fit the inside of the FP spring and the hole in the rear of the slide.
*In fact, a nail is a good place to start. Strength is not a factor and a nail can be chucked in a drill press and the mods done with a file. The length can be adjusted as necessary after the rest is OK.
Hope this helps.