pine tar goes on just like BLO or Tung, mix it 50/50 with turpentine or mineral spirits and apply (rub it in). One to two coats of the mix, or more, whatever your taste is. If you PM I can tell you where you could get a small bit of genuine pine tar.
Try stripping it with Lacquer thinner. All that old finish should wash right off. Use a piece of Scotch Brite to help loosen it up. They didn't all get the pine tar treatment. I've got a couple M39's with Tung oil on them. Most of the post war stocks didn't get the pinetar treatment. What ever you decide to put on it will be a hell of a lot better than Polyurethane. Yuck!