I just found one of these locally for $150. It appears from the photos to be in nice shape. I doubt that it is the trench gun version due to the fact that it has the frt bead sight and no markings ( the "P" and flaming bomb). It does appear to be a genuine riot gun (short barrel). I'll know when I go to see it tonight if the serial numbers match. My question is I've got very limited knowledge and couldn't find what I wanted to know on the internet, what is the rough value? I'll admit I don't want to keep it but rather trade it towards something I want. Is this worth it?

Thanks guys!

If you do decide to buy it make sure the firing pin is not broken.  I broke mine dry firing

and haven't been able to find one for many years.  Nice gun though.

I had one I picked up 20 years ago. paid 100.00 for it. It came with iron sights and a U.S. marked soft case.
Years ago i seen several P.D. trade ins. Some of those did have the front bead.
Numrich may still have parts for this. Good gun it was well built. I got 350.00 for it 10 years ago.

I had the same question because I bought one recently myself from my neighbor. I don't know the value because it was a distress/divorce sale and I got mine for $75, which I thought was a great deal. It's in really nice shape.  It seems like a very well built gun and mine operates perfectly.  Best that I can determine, mine was built in the early 20's and somebody took good care of it. I guess it's the riot gun version?  There's not a lot of information about the gun out there and I bet parts are hard to find.  

I can point you to a link that an informed gentleman on another forum sent me to for some more information. It won't give you a value, but it will tell you in detail of this particular  gun's fine reputation.  

(my 520 is the bottom one in the picture, does it look like yours?)

More info -> Stevens 520

http://www.quitthat.com/images/shotgunsweb.jpg