Neat lock! I'm not an expert, but it looks like it's pieced together. Pretty sure that you;re right- a half stock converted to full stock. It appears that the stock has been refinished, probably to get better color match. Trigger guard and forearm inlay(s) look out of place, like much later add-ons. I don't know, but it has a lot of attributes of someone's idea of a how to 'restore' an incomplete piece and 'improve' it to make it 'better,' at the same time. Then again, it's possible that some long ago owner was a making the improvements himself. I'm hoping that someone more knowledgeable than myself will chime in. All in all, you'll need more detailed photographs to gain a more specific opinion on this piece. Good luck! BTW, have you taken it apart?