That sling swivel looks very similar to the detachable ones that were standard with the German military Gewehr 88 and Gew98 rifles. These allowed to fasten the the rear end of the sling in two ways: At a hole through magazine box (M88) or trigger guard (M98) for parading or to a base under the buttstock for carrying. As Steyr made about 300K M88 infantry rifles for the German armies from 1889 on, it was a design well known by them in 1900.