LONDON.- An extremely rare so-called bullet coin the size and shape of a marble discovered in a collection of Far Eastern coins formed in the 1960s is expected to sell for up to £6,000 in a London sale next Wednesday (June 5).

Made in 1880, the silver coin commemorates the death of the mother of Rama V (1853-1910), known as the Royal Buddha, who was considered one of the greatest kings of Siam. It will be sold by specialist auctioneers Morton & Eden in association with Sotheby's.

Bullet coins (pot duang) remained in use in ancient Siam (Thailand) until 1860 when traditional flat coinage was introduced. They were made from bars of silver and gold, thicker in the middle, and bent round to form a complete circle. The shape is thought to imitate the ancient currency of cowrie shells, widely used as money. Their value depended on size, ranging from 1/128 of a baht to 80 baht in silver and from 1/32 to 4 baht in gold.

The very rare example in Morton & Eden's sale would have had an unusually high value of 20 baht but would never have been in circulation and was probably made for ceremonial purposes.

Auction specialist Jeremy Cheek said he was amazed when he found the coin in the collection. "Bullet coins of this size and type are extremely rare and unusual," he said. "The uninitiated would pass them by but they are coveted by collectors.

"They are always struck with countermarks to prove their authenticity, one of which is always an ornate chakra wheel. This example also has an ornate royal crown mark and an exceptionally rare mark showing a Thai flower with the date 1242."

Interestingly, the collection was also found to contain a pattern or suggested trial piece for a flat coin imported into Thailand in 1835 by a Mr Hunter to show to Rama III as a possible replacement for traditional bullet coins. It was not well received and the king sent him packing. The pattern is estimated at £800-1,200.