The second generation 9mm pistols have not garnered that much collector value as of yet. The second generation introduced a firing pin safety system that is often reported to have made the trigger pull heavier than the first generation pistols. As for value of your Model 439, it does have the nickel finish, most were blued, so that increases value. Yours also has the fully adjustable rear sight, again, that increases the value. And you have the box. I think $600 is reasonable.

Will the second generation pistols gain collector value? I believe they will. There are a lot more first gen pistols out there than there are second gen, so the really nice examples that are still in their boxes are going to become collector items.