That looks like a very nice early example of a second series Colt Challenger. The serial number dates to 1950, which is the first year of production. I believe production started in May 1950. From your pictures (I can't enlarge them) it looks like it has Coltswood plastic wood grained stocks that were only used for the first couple of years of second series Woodsman production, and would be correct for that 1950 Challenger (Don't quote me on that). From my experience I've found that 4 1/2" barrel examples are less common than it's 6" brother. The icing on the cake would be that it comes with the original box and contents. At $500, I think it's in the ball park, and I would buy it at that price as long as the other side of the pistol looks as good. I hope Mr Rayburn comes along and comments as he is the real expert. Looks good to me if you are looking for opinions!