The boxy thing on the front looks like it was designed to lock into something else... and the pistol part looks like it breaks open like a shotgun. The boxy part looks like it has a long horizontal opening in front.
I know that many aircraft engines back in the day used shotgun rounds to get them started. Could this be some sort of device like that?
ie: The ground crew loads a shell in the device, locks it into a port on the side of the engine, fires it off, and the compression starts the engine. After it's running, they then would remove it.