How many guesses allowed?
The second photo made me think of a gun mount, whaling harpoon or ship to ship line throwing? Last photo shows a Krag from the bottom.
Photo 2 does not resemble a Krag trigger guard strange hook near the screw.
Photo 3 Is that the same receiver? if so it is highly modified. The barrel has a hex near the receiver, to make it easy to change? Sight has marks for elevation that would not be compatible with a harpoon or line throwing. Sometimes used as a regular gun?
Combination harpoon and deck gun, that is my guess.