The Ithaca NID Magnum-Tens were built on a longer frame with an extension of the barrel lug through the bottom of the frame, and a larger diameter hinge-pin then the regular NID. All of the Magnum-Tens are in the 500000 to 501011 serial number range, and every one I've looked at is marked 3 1/2 inch chambers on the barrel flats.
FWIW -- The Chas. Askins/Elmer Keith original Magnum-Ten, serial number 500000 is coming up for auction in the March 2015 Julia's Auction. Of course it has the hideous Fred Shaw 1962 restock that Elmer had done and no longer looks like this --
