From the hammer style and the single line "MADE IN USA" stamping on the frame, it is no newer than 1947-48. The Model 10 terminology did not begin until the late 1950s. Those like yours are usually called a pre-Model 10 M&P. The SN will be stamped on the butt, also stamped on the bottom of the barrel and the rear face of the cylinder. It probably is worth in the $300 range. You can easily find approximate period-correct Magna grips on eBay, often in the $25-$50 price range. Although for shooting the Pachmayrs on it are perfectly OK.