The jacket looks late 1940's - early 1950's to me. Does it have shoulder pads? I agree that the Saks label is poorly sewn in but the jacket looks like it's very nice quality and the label matches the label in our label resource from a 1950's garment: I think it's very possible it's from Saks and the label was (poorly) reattached.
Here's a screenshot of a page from a book called The Eighty-dollar Champion: Snowman, the Horse that Inspired a Nation that indicates Saks did sell riding apparel. You have a lovely tweed hacking jacket!
