The Sub-target rifle (or gun) machine was the invention of Mr. Henry Havelock Cummings of Boston, Mass.  I have yet to find any exact instruction on how the rifle was attached to the machine's fixture.  Descriptions of its use and the (N.Y.C.) Public School Athletic League shooting program, which was a big advocate, can be found and read on-line. (Google search:  1. The Public Schools Athletic League, Outing 1908.  2.  Official hand book Public Schools Athletic League, Spalding's, published 1908).
  Wilkinson Sword Co. built the Sub-target device under license and advertised it in their  brochure of 1906.  The best descriptions are provided in the British materials, viewable on-line:  Google - Sub-target rifle shooting training devices - Rifleman.org.uk and select Wilkinson.
  BTW, the Sub-target machine, as manufactured by Wilkinson, was part electrical and utilized a 'dry battery' in its pedestal.  (Photo - a score card/post card from 1908 "School Boy Rifle Shooting Tournament" under Capitol Rifle and Revolver Assoc. and N.R.A. of America.  Note pin holes in target that were made by sub-target machine). 

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