If it doesn't say Lefever somewhere on the gun, I don't think you have a Lefever.
An early model Lefever single-barrel 12-gauge shotgun with a 30-inch plain barrel can be found for sale online for $300. This is the price for one that is in excellent condition.
Your Shotgun was made by Lefever from 1885-1919.Lefever started business in the year 1885.A (G grade) model hammerless 12 gauge shotgun will range in price from 800-1,400 dollars for a gun showing between 10%-70% of its original finish remaining,and a good set of bores.
The LeFever New Nitro 12 gauge shotgun was manufactured from 1936 to 1948. If it is in good condition, it can be worth about $400 to $500.
According to doublegunshop.com the year of mfg would be 1922
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.
Blue Book of Gun Values
The Lefever Nitro special shotgun was made from 1921-1948.During this time frame the lefever shotguns were owned by the Ithaca Firearms Company.The price range of your shotgun if it has between 60%-90% of its original factory finish remaining is from 300-450 dollars.I hope you find this info helpful.
Lefever made several models through three different versions of the company. Not enough information.
What kind of info do you need? please email me at email@example.com and I will try to help thanks Rick