Chris, as an example, Lodge made pieces into the 1990's that were never stamped with Made In USA. The lack of it does not always give an indication of its age. A similar argument can even be made about dates given to pieces with gate marks. And of course look at all the Asian cast iron today that isn't even stamped for its country of origin. Most of it is only mentioned on the packaging now.
Dwayne is correct. You can even look up the patent for this pan on the US patent website, but that patent number escapes me at this time.