Mark, I don't disagree that if the words "Made In USA" are on a piece of cast iron cookware, it is likely that it was made post-1960. That's not the same as saying it was required that they had to be marked that way by regulation.
Since we all know that anything you find on the Internet is correct ;), it's important to be careful to pass along information that is as accurate as possible.
Why would the makers put these words on a piece? My first reason would be for export purposes, but if you sit and think about it, other than possibly to Canada and Mexico, I suspect the export market is small. Instead I suspect we see that to advertise the items as a US made piece to allow customers to buy American rather than one of the flood of imported goods that were/are coming in. Just speculation, not fact though. :)
How did people put a 1960 date on the phrase? By looking at the casting design, style and process. Looking at logos, catalogs, and shipping materials, even contacting the makers (they don't always know though.)
Do I have examples of the phrase on earlier cast iron pieces? Not off the top of my head, but I'll remember to look. Might take a few years though.
Just for fun, here's a picture of a 1926 Ford hubcap with the phrase.
Regarding Castelain, I did a search on the US Patent and Trademark site without any hits. There is a nice blond beer trademarked by that name though :D.