I wish some of these photos and drawings showed better detail of how a rifle was 'hooked-up' to the sub-target rifle machine.  As well as, how the linkage connected the trigger/striker of the rifle to the rod that marked the "calling card" sized target to record consistency and steadiness of position, aim, and trigger squeeze.  (It utilized a cable, I believe).
  Use on a 'sub-target rifle machine' is the most logical explanation, I can come up with, for the 'stock-holes' and 'lobe on the striker' present on tbergdall's .22 cal. Krag.