As stated by others you have a12G gun bearing proof marks in use between 1875&1887.The gun was made/shipped in 1876.
The 3084 # relates to a patent issued in 1873 covering Greener,s famous cross bolt action.
On the tang of the trigger guard there may be some letters or combination of letters and numbers that may give a clue as to the original grade and price of the gun.
Mr. Hebbes I have a question for you unrelated to this thread but concerning Greener guns. If you have a chance please PM me or E-mail me. Thanks!