The A5 is an excellent design and a well made gun. There are, however, far more such guns available than there is demand for them. This is almost entirely due to their unsuitability for use on migratory birds. This fact severely limits the market for A5's because it was this purpose for which the A5 was especially well-suited.
They were used to great effect in the uplands, too (and still could be), although most upland hunters would probably opt for a lighter gun these days.
Your A5 is a mid-grade example and is worth what you paid for it as a collector's piece. But, you didn't buy it solely to admire it. Once fitted with choke tubes, it will once again be a fine waterfowler. IMO though, having done this work will not increase the value. In fact, shooting this gun will inevitably result in minor dings and scratches, as well as wear to the bluing and wood finish.
When the day comes to sell, the gun will have been modified (reducing the collector value) and will be reduced in overall condition as well. IMO, the only way you'll get what you paid for it, is if you sell it well into the future, when the purchasing power of $500 is half of what it is today. So, hopefully you didn't plan to turn it over for a profit some day.
Having said that, there are boundless purposes for a nice shotgun besides generating profit. If you bought it intending to use it, my advice would be to go ahead with installation of choke tubes and get as much enjoyment out of it as life will allow. Look after it, but don't hesitate to use it in the mud and the wet. Forget about getting your money out of it some day - instead, look forward to leaving it to your kids, or your grandchildren, because with even just basic care it will surely last that long.
Flipside: If you expect to get all or most of your purchase price back, I'd suggest the only way that will happen is if you sell it just the way it is today to someone who wants it for the condition it's in (and, obviously, plans to keep it original).