Agreed, I'd leave it alone. Sanding always makes the appearance of an honest beat up stock worse since it's so obvious, resulting in blurred stock serial numbers, more rounded edges, etc. Your pictures are rather low resolution but I don't see the "thick" appearance of polyurethane. Probably linseed oil or tung oil (or more likely the petroleum distillate diluted and drying-catalyst added versions commonly found on the market, e.g., boiled linseed oil, or the tung oil products found in hardware stores). In my opinion, tung oil and linseed oil are the least objectionable finishes used by those who feel compelled to try to shine up a beat up warhorse. Applying shellac over dents, gouges and scratches generally doesn't turn out well unless the blemishes are minimal.