Fred, great job on finding the correct Patent going from the date of application.
So it appears this tool is meant as an alternative to a roll crimp, and from the Patent description, it seems it is intended for paper shell cases. As Stan points out, it probably wasn't a rousing success, and it certainly didn't edge out other methods of closing the end of a shot shell.
It would be interesting to see it actually used to load some shells. I would assume that an over-shot card is used, and the tool creates a "crease" to hold it in place. I'd also assume that a mere "crease" would not be as effective as a good roll crimp or a conventional 6 or 8 point crimp, neither in keeping the shell assembled without losing shot, or in providing enough resistance to get complete and reliable ignition. I can imagine that shooters who used this tool might have resorted to things like using water glass (sodium silicate) or melted candle wax to keep the over-shot card wad from becoming dislodged.
Another thing I found interesting from Fred's Patent search for the date April 6th, 1875 was the number of other Patents applied for on the same day that were related to firearms or reloading cartridges.
