An Australian 1921 cupro-nickel pattern penny is expected to see strong attention from collectors when it appears as part of a March 26 coin auction in London.

The unusual square coin is expected to see bids in the region of £25,000-30,000 ($37,500-47,500).

Many of the square pennies on the current market are those that the treasury officials failed to return to the mint

Between 1918 and 1921, the Australian government toyed with the idea of replacing the existing one penny and halfpenny coins with a smaller and lighter coin. As such, a series of 13 different designs were struck as trial patterns, with the majority made from cupro-nickel, an alloy of copper and nickel with additional strengthening elements.

These coins all had different portraits of George V, with a kookaburra design on the reverse. They were issued to treasury officials and leading figures for evaluation, but the plans were soon scrapped after issues with vending machines.

There are only a scant few examples available on the current market, many of which were those that the treasury officials failed to return to the mint. The example at auction was designed by Sir Bertram Mackennal and is in superb, uncirculated condition.

In January 2012, another 1921 example sold at auction for $57,500, while in 2008 a Sydney auction house sold four examples from the Wright Collection, each making between $42,000 and $44,000.

With a wide selection of world coins available, the second highest bids of the present sale will be seen by an AV Mohur from India's Mughal Empire (1605-1628), which is estimated at £20,000-25,000 ($30,000-37,500).

In May, Baldwin's will offer the outstanding David Fore Collection of British Indian coins - one of the finest collections of its kind.

Paul Fraser Collectibles also has a sterling selection of coins for sale, including rare Italian renaissance pieces and spectacular British examples.