The workings of this device have always been kind of vague to me. They seemed connected to sight picture/dry fire practice with the 1903 Springfield and 1917 U.S. Enfield rifles. But, I didn't give them much attention.
In the last few days, I saw a question on another forum about the peculiar practice "Dummy Cartridge" used with this device. I also saw on ebay a partial Hollifield set for sale. This encouraged me to try and connect some 'Dotters'.
Although, this Hollifield set has graphics showing use with a 1903 Springfield and seems to be lacking its "Dummy Cartridge", it has booklets of scaled-down practice targets labeled "Krag Targets".
This leads me to think that maybe some of these devices had "Dummy Cartridges" and barrel "practice rods" sized for use with the Krag rifle.
The Hollifield Target Practice Rod Co. was in business from 1908 to 1927. Their device used a fake cartridge, that when 'dry fired', would transmit the movement of the firing pin to a barrel rod. The barrel rod concealed a needle that "darted out" to pin prick a miniature target.
The target card actually had two superimposed "bulls". The card was put in a holder (attached to a wall or the factory box) just beyond the rifle muzzle. The 'Shooter' aimed at the upper bull. When he dry fired, the needle would puncture the lower bull to record his 'shot'. Good consistent shooting practices would result in a tight group of pin pricks.
Some Hollifield ("Dotter") device pictures:
