Back in the 1960s Ithaca Gun Co. did a tabulation of the NIDs. It showed a total of twenty-two No. 7E NIDs. They broke out the smallbores and the Magnum-Framed guns, showing two .410-bores, two 28-gauges, two Magnum-Twelves and one Magnum-Ten, so that leaves fifteen to be divided up between 12-, 16- and 20-gauges.
First one I ever saw was a 20-gauge at Kerr's Sport Shop in Beverly Hills in early 1970 when I was a U.S. Navy E3. Pretty sure it is the one that showed up in Julia's Auction in 2014 --
http://jamesdjulia.com/item/2459-369/ 
