Fascinating piece, CC! I can't say I've seen anything like that before. The piece looks so 'minty' I have trouble believing that its of the 19th century. Love the haft with its patterning. As you pointed out, the langets are not in the typical French positioning (front to back with the blade/spike). The hook could have been used as a gaff, I suppose. The form looks to me more like the trench axes of the early 20th-
If it is a boarding piece, I agree with you that it most have been an unproduced prototype.
Mark