Nitro in the name would imply smokeless powder, so the earliest would be shortly after 1900. Could have been as recent as 1945.
A75183
20004 to 2007
nitro hunter shotgun single barrel with exposed hammer stamped belknap hdwre and mfg serial 5516
It's part of the serial number.
No published sn data that I know of.
I would say that with the serial number 1321XX,we are looking at your shotgun being made by ithaca(who made the lefever nitro special) in 1924.
CHeck the receiver and under the forearm
1929
Ithaca Firearms manufactured the Lefever Nitro Special shotgun with serial numbers between 101600 and 119899 in 1922.
The best that I can say is that the Lefever Nitro special was made from 1921-1948.I do not know if the serial numbers to year of production is available in the public domain.
70-100 years
Seldom exceed 150 USD