Try some paint stripper. The factory finish is likely long gone, so it's doubtful you will damage it. I have used it to strip various items with several layers of paint with good results. If someone went through the trouble of painting it, it has probably already seen sandpaper, and I don't have any idea as what will remove paint yet leave the varnish intact. Unless you plan to refinish it yourself, why not just leave it black? It doesn't look bad, and maybe buy an original stock at some point that has a good finish? It already has mismatched parts, so a new stock isn't going to hurt. If not, just strip and refinish what you have.