The Second Edition of "The Krag Rifle Story" has additional details (Mallory, page 196) on Sub-Target Gun Company's device.  The bronze 'fixture' that attaches to the right side of the rifle seems consistent with the stock inletting on 'tbergdall's' Krag.

Dick:  Rear part (sight notch) of 1902 sight extends a lot further to rear than 1901 sight.  (Only 5/16" short of rear rivet of hand-guard).  I don't think there is much of an anomaly.

This is really quite an intricate device.  The bronze 'fixture' or armature, that attaches to the rifle, has pivots on its ends that link it to the machine.  The cylinder, which holds and marks the miniature targets, apparently lies on the machine 'bed' and is actuated by a cable.  (I've since learned, on some machines, the release of the trigger completes an electric circuit.  There is a dry battery located in the machine's pedestal.  An electro-magnet propels the sub-target into a pointed rod to be pierced, thus recording the 'shot'.  This is reported in the 1906 Wilkinson brochure).

I suspect the 'hay day' of the sub-target rifle machine was 1907-1908.  By 1908, the Public School Athletic League of N.Y.C. had put one in each High School, (reported variously as 11 or 12).
  Denise Conni, (N.R.A. Marksmanship Qualification Program Coordinator), in "100 and Counting" reports on early PSAL-NYC activities and competition with .22 Rifles and the ..."sub-target rifle machine -- a marksmanship training device with more than 300 parts and a lofty 1904 price tag of over $200..."
  I would guess over time the sub-target rifle machines required maintenance, adjustment, and repair.  'Virtual Shooting' and one machine per high school probably lost out to the economy and match environment with .22 target rifles.
  Possibly, 'tbergdall's' Krag, converted to .22 rim fire, is testimony to what happened:  An expensive training device probably lost out to target rifles that were practical, fun and inexpensive to shoot indoors.

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).