The item was shown to me by Louis Hudson.  It is 23.8 grams, and has obviously been cut off the end of a bar.  The Roman numerals XV and the three dots indicate 15.75 carat fineness. The crowned C must refer to Charles I of Spain, who else?  The item was recovered off Grand Bahama in 1992, from the wreck of the Tumbaga.  About ten other gold pieces were found with it.  Many were not marked at all, but some were reported to bear bear assay marks indicating fineness as low as 9 carats. Pradeau mentioned a 1528 statute requiring the re-assaying and stamping of circulating gold with the weight and fineness along with the royal arms and the motto PLUS ULTRA.  This piece, then, would be legal according to the 1526 statute, but not that of 1528.  It can be nothing other than an example of the first gold coinage of the Americas.  Under the royal stamps is Montezuma's treasure.

    This is probably the most exciting piece of numis I have ever held in my hand.  Here are two pieces of writing about it:
The description from Dan Sedwick's catalog when he owned it, along with another article about similar items from the 17th century
My recent article on the conquest of Mexico from the May, 1998 World Coin News

Although I don't own the item, it is for sale.  Get in touch if you're interested and well to do.