I wouldn't mind having that gun in hand because from the photos I am not 100% sure it has been refinished. I know that the inside is not all "in the white" but I have seen some original pieces where the rust blue has bled down over the receiver rails, and the frame on this one looks like old grease and age darkening. Would want to assess it carefully before writing it off as a reblue.
The magazine has a bit of a mystique with that marking, but as Tom said, it has nothing to do with US Ordnance (it is a flaming bomb and not a Taurus symbol...although the Taurus symbol is also found on some Lugers as an inspector's mark). Some collectors like them because they think they are "proper" for a test piece, but a plain unmarked base is just as proper. They do command a premium, but I suspect $500 would be toward the high end (more of course if it is mint). The base on Mike's magazine looks in very nice condition but does have the hairline split where it is pinned to the mag body, so care should be exercised to not cause further damage (don't load it or drop it!
) since it is quite collectable.