Long Branch rifles were not made by Savage. 'Long Branch' was the name of the Canadian Arsenals plant in Long Branch, Ont. Currently Mississauga, Ont. a suburb of Toronto. However, once the contract between the Brits and Savage was finished(June of 1944), all remaining rifles, parts and machinery got shipped to Long Branch and issued to the CF. Had one on my MIU long ago.
As to value, it depends on condition if it's still in full military configuration and if the headspace is good. (Thousands of 'em were assembled out of parts bins with zero QC. Not even ensuring the headspace is good there by making the rifle safe to shoot. This does not mean that particular rifle is unsafe. Just means you have to check the head space.) Anyway, $315 isn't too bad depending on all that. Where you are matters too. They start at around $500US on Gunbroker.
1943 is the manufacture date. There is no data base of S/N's by month. There are some guesses though. Best guess is 57L numbered rifles were made in December of 1943. The knowledgeable guy who did the guessing posted his guesses here.
http://www.milsurps.com/showthread.php?t=53153
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