Hello, welcome to Just Answer and thank you for your question!
First off, your gun was actually manufactured in 1917 according to the serial number you provided. The date "May 1903" you find on the gun is most likely the date of patent. It is impossible for me to give you an approximate value on your gun for a couple of reasons: 1. I don't have enough information about your gun (i.e. "grade", with/without ejectors, barrel material). 2. Parker guns, in many cases can be very valuable, and because of that it is best that you seek a professional opinion by showing the gun to a expert/collector in person. That said, I can offer some helpful things that will help get you "in the ballpark". Unfortunately, shortening the stock almost certainly devalued it. Collectors are normally looking for items that are as close to original as possible. To find out the grade of your gun will give you a much better position to find out value. If you know how to disassemble the weapon then you can find the grade, which will be engraved on the water table. If you are unfamiliar with how to do this, you should take it to an expert or gunsmith who knows how to handle a historical gun like this.
Go to www.parkerguns.org . Here you will find a great deal of information regarding your gun. On the menu at the left, click on "parker grades" to see explanations of each grade. Also click on "technical information" to see where the engravings lie on the water table. Once you have determined your grade and barrel material you are in a much better position to determine its value. Parkerguns.org has a great forum where you can post pictures and information about your gun and garner some expertise from experts
and collectors of Parker weapons. They might be able to give you an approximate value, but will probably also suggest you seek in-person advice. They would probably know of someone in your area who could take a look at it.
Also, if you haven't already checked out what similar guns are going for at auction, check this link: http://www.gunbroker.com/Auction/Browse.aspx?SearchType=0&Timeframe=0&Keywords=parker&Cat=3104&Items=50
I have it narrowed down to side-by-side Parkers, but you can see the wide range of valuations based on grade, date and condition. Hope this helps, sounds like you have a great gun! Best wishes!
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