If this has already been mentioned, then please forgive the post. but I have been away for a significant period of time and may have missed it being mentioned. A search did not turn up any reference to it.
I recently discovered an auction on ebay of an admittedly fake enamel canteen and cup: the owner of the auction mentioned outright that it was indeed a repro done for re-enactment purposes, but since he is using original (uncoated) canteens and cups, some may be fooled into a bad investment by a seller of lesser character. The US stamp is quite obviously wrong to the experienced collector, but a new collector may be fooled by it, if misrepresented in a sale.
Auction link is here: http://www.ebay.com/...cvip=true&rt=nc
"Caveat emptor", indeed. Again, please forgive me if I am "late to the party" mentioning this.