Thanks, that clears up the Proof stamp question. Based on the information you have given me this is what I can tell you.
The serial number was stamped on your rifle's receiver during May of 1915. If you would like to know the exact date you can contact the Cody Firearms Museum. It is typical that it would take a month or maybe two before the rifle would be fully assembled and in the Winchester warehouse.
The barrel address style on your rifle is the most consistent style found on the Model 1892 rifle. It was used on almost 700,000 of the +1,000,000 rifles manufactured. The upper tang style on your rifle was used on about 340,000 rifles and change around SN 830,000 by adding "& FGN" to the lower line.
If you can get somebody to take photos of your rifle and forward them to me I will be able to assess the condition and originality of the wood and metal finish and an approximate determination of the value of the rifle. If you have any further questions please do not hesitate to ask.