Xausa: No, it is a windage-adjustable Parker-Hale "Sportarget" peepsight, see the 1940 Shooters Bible p.209. It is indeed brazed to the standard M-S firing pin nut. Better than the Lyman #1 on my Gibbs, Bristol M95 6.5x53R aka .256 Mannlicher, as it clicks for both w & e. It came on the rifle. On other rifles, I have substituted these by altering another P-H model, the #16 made for target rifles once and available at gunshows sometimes, same elevation stem, and brazing those to Mauser and M-S nuts. On my Gibbs the Lyman sight is mounted in a dovetail filed into an original nut with the firing pin shortened accordingly. Same for a Rigby peepsight on the long nut of a commercial Mauser.
Lancaster: IMHO you are only right in part. There was an identical case before 1910: In the 1904 DWM catalog the 9.4x56 aka 9.55x56 is listed, case #393. A very heavy 25.7g =396gr rn solid bullet #156 of .378" diameter is listed for this case.
There may have been other reasons for Steyr to go to a 9.5mm cartridge: They needed a proprietary chambering of their own, competitive with their German competitor Mauser's 9.3mm'rs. Of course, it had to be different enough, so that it was not to compared not as easily as their former 9x56M-S and 8x56M-S offerings that were obviously inferior ballistically to the similar-named Mauser cartridges. They needed such a cartridge not only for export, but also for domestic use. Remember, in 1910 the Austro-Hungarian empire spanned from (now) Italy to Ukraina and Poland to Bosnia. It included the wildest and best hunting grounds in Europe, like the Carpathian mountains, thrieving with bears, huge stags and 800 pound wild boars. As the 9.5x57M-S is a little bit "light" for really big African game, I think they intended it for use on their domestic big game.