William Brophy put a very clear photograph of the 'blank device' in his book, "The Krag Rifle", page 135.

From what I gather: (The original blank cartridges for the U.S. Krag, had a 'paper bullet' containing a black-powder charge. This paper projectile allowed feeding through the magazine and the paper bullet disintegrated upon firing, as a safety measure.
The pre-WW1, M-3 blanks were shorter and without projectile. They required the bronze device and some alteration of the follower in order to feed through the U.S. Krag).

The device, Brophy described, was nicely made, held in place by a Screw through the side-plate, and serial numbered to match the receiver.

(I am 'Persona Non Grata' with Mr. Farmer. Perhaps someone, who has his book and access to his site, can review his materials or ask him).