I would also tend to suspect it was originally a target rifle that has been shortened. As to its current form I have heard such short barreled guns referred to as "buggy rifles" in the past (fit easily under a buggy seat), perhaps a way to salvage a gun with a damaged barrel. Though it could possibly have been made this way if someone felt the need for such a piece, the sights could have been set as they are to give the longest possible sight picture with that short barrel