I think Stevens/Savage was no longer using the Eastern Arms alias when they started the date code system. I found one reference which states that the Stevens 311 was introduced in 1931. Not sure whether the 5100 came before or after.
I have my father's old Eastern Arms singleshot .410, on which is stamped "1929 Model". So Stevens was producing Eastern Arms guns at least that early. I'd guess late 20's-30's for the 5100 you're looking at. And from your description, agree with Rockdoc that $300 sounds like a reasonable price.
