Early on, Ithaca Gun Co. used the Miller Single Selective Trigger on their NIDs. The list price was $30 from 1926 to 1932. For 1933 it went up to $32.45. Then Ithaca's designer Harry Howland got a Patent to get around the Miller brother's and by 1934, Ithaca Gun Co. was doing their single selective triggers in-house and the price dropped to $21.60 where it stayed thru 1939. Probably best to contact these folks --
http://www.millersingletrigger.com/services.html
or
Gunter Pfrommer in Rocky Mount, Virginia.