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The Lefever was mfg. in 1935.
If it is a double, remove barrel and you will see it along with serial #.
I can say that the LeFever Nitro Special was made from 1921-1948.I do not know if the serial numbers by year of production are available in the public domain.You may have the Grade A field model due to the engraved ducks on the side.These were made from 1934-1942.Barrel length was available from 26in.-32in,of which your shotgun fits.
around $6000
1927
Requires professional appraisal.
Turn of the century.
The best that I can do is say that the Lefever Nitro special was made from 1921-1948.
do you have a picture
Blue Book of Gun Values
To get a parts breakdown you would need to take the shotgun into a gun store. The store will be able to provide an estimate, or order new parts.
Break open the action and it should be engraved on the metal that is normally unseen when the action is close, that or it should be on the barrel. If the chokes are removable then it should be engraved on those as well.