The pics of the cartridge indicator look strange to me, is there one there or not? Looks like it may be broken.
Wonder if you can dremel out the hole for the slide stop on a regular 1907 mag?
There definately is a loaded chamber indicator, but the tip of the arm looks a little short, which could be normal variation (the tip just folds under) or broken off.
The 1915 mag is a modified 1907 mag, so it is possible to open up the side of the body for the tab on the 1915 follower, which is what trips the slide stop.
When I have the time, I want to try modding some aftermarket 1907/1917 mags to 1915 form. While 1907/1917 factory mags are much more common than 1915 mags, they are still hard enough to find (and expensive enough) that I hate to experiment on them.