From your pictures it appears that the trigger is in the righthand slot and the selector in the left. When A.H. Fox Gun Co. finally brought out their Fox-Kautzky Single Selective Trigger in 1914, they did it the other way with the trigger in the left slot and the selector in the right.
21987 03.jpg
Joe Kautzky had at least three patents for single triggers -- No. 827,242 granted July 1, 1906; No. 990,562 granted Apr. 25, 1911 and No. 1,109,632 granted Sept. 1, 1914.