Paul, your serial number dates it to 1940. However you stated type as FWSP. If the serial number on the receiver flats is FW S181513, you added one too many numbers. The prefix "S" would mean it was a Marlin gun starting in 1946. In 1968 it would have a stamp of FW SM, and then the serial numbers didn't go that high. I don't know what the P in FWSP indicates. Post some pictures. I am assuming that the condition must be like new if it is in the box. Price would be high for in the box condition. I saw a Field Grade 16 ga. FW S17xxx go for $3700. A 12 ga. could go that high especially if older and in the right place.