Look under the rear sight, in front of the serial number, for the actual maker of the receiver/carbine. The value often depends on how original it is. Some of the different USGI carbine manufacturors used their own barrels, and some used other maker's barrels. If your is the original barrel, receiver, and bolt combinatoin the value goes up a bit. But you are probably looking in the area of $500 - $600. The actual maker of the stock(providing it is a WW2 stock) will be ID'd by the letters stamped in the left hand sling cut out. Sometimes they are hard to see. Bolt ID is usually stamped on the end of one of the locking lugs. Slide will usually be stamped inside. More than likely the SA/SHM is a rebuild mark put there by Springfield Arsenal when they rebuilt the carbine.