The undated (1538) silver 8-real coin is estimated to sell for between $500,000 and $1 million. The opening bid is $475,000.

Numismatic Guaranty Corp. has graded the Sedwick example EF-45.

All three known examples of the coin were struck at the Mexico City Mint, under Francisco del Rincón, the facility’s first assayer, just two years after the mint opened as the first in the Americas.

In the early 1990s, these coins were discovered in a chest of 2,000 coins from a circa 1550s Caribbean shipwreck, according to Daniel Frank Sedwick, the company president. The discovery provided physical evidence of the early issues, which were reported in documents transcribed by researcher Alberto Pradeau in 1947 and supplemented in 1955 by Robert I. Nesmith.

The transcripts were from an investigation by Francisco Tello de Sandoval in 1545, as ordered by the king in response to accusations of fraud leveled by Hernan Cortés, Spanish conqueror of Mexico. Though the large-sized early coins were referenced in the report, no extant examples were known prior to their discovery in the shipwreck.

All three examples of the coin were struck at least twice, confirming Pradeau’s research that found production of the coins was halted because of difficulty with striking.

The coins share the same basic designs.

The obverse features a crowned shield housing castles and lions in its quadrants, representing Castile and Leon, with a pomegranate for Granada at the bottom, flanked by Gothic-M Mint marks for Mexico inside a legend that shows the name of King Charles and his mother Joanna, the “mad” queen.

The reverse of the coin bears the Pillars of Hercules with a banner that shows the inscription PLVS VLTRA (meaning  “more beyond”), signifying the entrance to the Mediterranean. A Maltese cross appears at the top, and an initial R for the assayer is at the bottom. The various design elements are all contained inside an encircling legend stating the rulers’ territories.

This is the first public offering of this example.

Other examples

Another example described as “pleasing” Extremely Fine realized $373,500, including the 15 percent buyer’s fee, during a Jan. 9, 2006, auction by Heritage Auctions. According to Daniel Sedwick, the price was tamped down by speculation that more examples would emerge.

The third example was sold May 26, 2008, as part of the Millennia Collection by Ira & Larry Goldberg Coins & Collectibles. Graded EF-40 by NGC, it realized $310,500 with a 15 percent buyer’s fee.

“We cannot overstress the importance of this auction, both for the history of Mexico and the larger field of North American numismatics,” Sedwick said. “These coins are the absolute root of the concept of the American dollar.”

Agustin Garcia-Barneche, the vice president of the Sedwick auction firm, drew several parallels between the early Mexican 8-real coin and the American silver dollar, noting that the U.S. 1794 Flowing Hair dollar (which weighs 26.96 grams) was based on the Spanish colonial 8-real pieces like those struck at the Mexico City Mint and elsewhere in Spain and the New World for some 300 years.

“Spanish colonial coins, particularly from Mexico, were legal tender in the U.S. until 1857 and therefore the first ‘dollar’ of Mexico is technically the first such coin of the United States as well,” he said.

The auction is scheduled at the DoubleTree Hotel Lake Buena Vista at the Walt Disney World Resort.

Printed catalogs will be available in early October.

For details about the auction, including hotel reservations, telephone the firm at 407-975-3325, email it or visit its website.