I suspect a letter from Cody is the only way to know when an Ithaca NID Magnum-10 was made. From my observations I've seen features that don't match up with the chronology. I don't believe that sales of these very specialized guns took off very fast being introduced right into the very depth of The Great Depression. Here is the introduction from the second Ithaca Gun Co. 1932 catalog --
Magnum-Ten Announcement 1.jpg
Magnum-Ten Announcement 2.jpeg
and an ad from the July 1932
National Sportsman --
July 1932, National Sportsman, Magnum 10.jpeg
Capt. Chas. Askins did articles "American Magnum Duck Guns" in the August 1932
Outdoor Life and "Magnum Shotguns" in the July 1932
The American Rifleman.
Capt. Askins apparently didn't keep his Magnum-10 serial number 500000 very long as Elmer Keith did an article "The Ithaca Magnum Ten-Bore" in the September 1935
The American Rifleman. Thirty years later, Elmer wrote "Long Shots and Far Away!" for the December 1965
Guns & Ammo. By then serial number 500000 had the hideous restock that it still had last time it was on the market.
500000 06.jpg
Col. George W. Busbey wrote of his adventures with a Magnum-10 in his article "Magnumitis" in the
1957 11th Edition Gun Digest.
The Magnum-10 I use to own had a seven-slot Jostam Anti-Flinch. I'd suspect that the big 15-heart Hawkins repros would work well. The Ithaca bridge trestle pad at the time was no offered in a size large enough for these big guns but Kings Ferry made repros in a larger size.