I had one for a while. Great gun. Perhaps, and arguably, one of the best double trigger guns available for CAS. Any of our talented 'smiths that work on SKB's will be familiar with the 500 and can make it race ready. It is proof, nickel chromium steel so it's as strong as any other double gun you might find -- which is to say plenty strong for 3+ dram loads (which we don't use in CAS out of concern for our hearing).
You should be able to get it for a very good price because: most people don't know what they are, there is little demand for double trigger guns, and it will likely sit on the shelf for a long time before another interested buyer comes along. But, for our game, it is worth a lot more than book price IMHO.
The chokes will be marked on the breech block, beneath the opening to the chambers.