Sorry for replying to such an old thread, but there seems to be no answers to your question and I searched your recent posts to see if you were still active and you are.
From what I have researched (read googled) your shotgun was made sometime in 1949. As for the model number, I cannot say for sure. Some sources (again, from google, take with a grain of salt) say the numbers such as 315, 5100 and 311 are the action types, some others say it is the model number. Unfortunately I cannot find where I read this info, so I cannot direct you to a source. Hope the rather limited info I have given you was helpful.