I've seen three Ithaca NID Magnum-Twelve doubles. There was an NID Grade 2E, 12-gauge, 32-inch barrel, 3-inch Magnum at the Cabelas in Sydney, Nebraska, November 2002, for $1299. It was serial number 500,144. The gun had a beavertail forearm, and at some point it had been back to Ithaca and fitted with their after-market vent rib with a ramp to fit on the standard rib frame. It also had a built-up Monte Carlo to go with the higher vent rib. I was just about to take on this "project gun" when I noticed a fairly good bulge just behind the choke in the left barrel, probably from using early steel shot shells.
Some years back, Steve Barnett had this refinished Grade 1E Magnum-Twelve, serial number 500155, at The Vintage Cup and Sandanona --
Both of these guns had serial numbers much lower than when one would expect Magnum-Twelves, as Ithaca Gun Co. didn't catalogue them until 1937 when they dropped the Super-Ten. Also, on the subject of Magnum-Frame serial numbers I use to own Field Grade Magnum-Ten 500,167 which was the new style Field Grade without cocking indicators and the border engraving that was introduced in 1935.
In August 2011, I examined a pretty high condition Field Grade Magnum-Twelve NID at the Big Reno show and it was bulged behind the chokes in both barrels. Can't lay my hand on the notes for its serial number.