It's somewhere between 57 and 58. Guns have set on the Savage shelves for over a year before being sold and it's common for inspector marks to be off. Actually 1957 and 58 rifles are pretty easy, get into the 500 and 600 serial range and they're all over the chart.
Best place to find a correct sight is Ebay, Royals book probably has a pic of the correct one (not sure I don't own the book) but Numrichs is wrong on alot of their sights.
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