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.
The Lefever Nitro Special was made by Ithaca. Yours was made in 1923.
The Ithaca Gun Company acquired Lefever Arms in 1916, and manufactured the Lefever Nitro Special between 1921 and 1947. The serial number on your gun indicates it is an "A Grade" made in 1937.
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.
That Lefever was made in 1923.
Ithaca Firearms manufactured the Lefever Nitro Special after it purchased Lefever Arms Co. in 1916. This model was made between 1921 and 1947; the serial number indicates yours was made in 1929.
Ithaca Gun Co. manufactured your Lefever Nitro Special in 1940.
Ithaca Firearms manufactured the Lefever Nitro Special with serial numbers between 100000 and 101599 in 1921.
I think that it was 1929.
It is found under the breach below the barrel