Nick, The only real way to tell what you have is to look at the receivers serial number, the barrels serial number and the forends serial number and look under Shotguns on the left side of the Home page. When your cursor reaches it, it will open and then go to Year of Manufacture and look up the serial numbers and see what you get. On the receiver if it was a Specialty Grade it would have either FW XXXXXX then underneath should be an "S". If there is no engraving on the side plates or any on the receiver it might be a Field Grade FW xxxxxx Field and so on. You knew that it was a Specialty barrel by the writing on the top. I don't think a letter would help unless they accepted the different numbers and it might be from three different guns. As for being a "lunch box special", any body's guess.
Also their were 1106 20 ga Specialty's made, and the additional 109 were the ones made from 1946-50 with the prefix of FW Sxxxxx