Question: The smallest denomination of the Hong Kong dollar is the one mil coin which has which identifying feature?
Context: The five dollar coin is the second-highest denomination coin of the Hong Kong dollar.  It replaced the five dollar banknotes in 1976. The one mil coin was the smallest denomination of the Hong Kong dollar from 1863 to 1866, after this date it was no longer issued but may have circulated much longer.  Its value was one tenth of a cent, or a thousandth of a dollar.  Despite being minted under British rule, they did not feature the reigning monarch as all other coins did, due to the hole in the middle. The twenty-cent coin is a coin of the Hong Kong dollar.  It is the second-smallest denomination coin in Hong Kong.  There have been two different periods of usage for a twenty-cent coin: the first, round twenty-cent was issued from 1866 to 1905, and the current scallop-shaped coin was introduced in 1975. The ten-cent coin is the lowest-denomination circulating coin of the Hong Kong dollar.  With a diameter of 17.5 mm and a mass of 1.85 g it is also the smallest in size and weight.  It is the oldest coin denomination to still be in circulation in Hong Kong.  Since its first issue in 1863, there has only been one period (1942–1947) where the coin was not in use. The Hong Kong dollar (; lit.  "Harbour Money"; sign: HK$; code: HKD) is the official currency of the Hong Kong Special Administrative Region.  It is subdivided into 100 cents.  The Hong Kong Monetary Authority is the governmental currency board and also the "de facto" central bank for the Hong Kong dollar. Linked exchange rate system is a type of exchange rate regime to peg the exchange rate of a currency to another.  It is the exchange rate system implemented in Hong Kong by Honorary Vice-President at the University of Hong Kong, Professor Y.C. Jao, to stabilise the exchange rate between the Hong Kong dollar (HKD) and the United States dollar (USD).  The Macao pataca (MOP) is similarly linked to the Hong Kong dollar. The two dollar coin is the third-highest denomination coin of the Hong Kong dollar.  Since its introduction in 1976, it is one of two circulating coins to not be round (the twenty cent coin is also not round).  The two dollar coin is a dodecagonal scallop in shape, and made of cupro-nickel. The Clearing House Automated Transfer System, or CHATS, is a Real Time Gross Settlement (RTGS) system for the transfer of funds in Hong Kong.  It is operated by Hong Kong Interbank Clearing Limited, a private company jointly owned by the Hong Kong Monetary Authority (HKMA) and the Hong Kong Association of Banks.  Transactions in four currency denominations may be settled using CHATS: Hong Kong dollar, renminbi, euro, and US dollar.  In 2005, the value of Hong Kong dollar CHATS transactions averaged HK$467 billion per day, which amounted to a third of Hong Kong's annual Gross Domestic Product (GDP); the total value of transactions that year was 84 times the GDP of Hong Kong. The Australian five-cent coin is the smallest denomination coin of the decimal Australian dollar introduced in 1966.  It has been the lowest-denomination coin in Australia since the withdrawal of the one-cent and two-cent coins in 1992.  Due to inflation, the purchasing power of the five-cent coin continues to drop and currently the coin represents less than 0.3% of the country's minimum hourly wage for workers age 21 or over. The New Zealand one-cent coin was the smallest denomination coin of the New Zealand dollar from the currency's introduction in 1967 to its demonetisation, along with the two-cent coin, on 30 April 1990.  With a diameter of 17.53 millimetres, it is the smallest coin ever issued of the dollar, and at 2.07 grams in mass the lightest as well.  Its reverse featured a fern leaf, a sign of New Zealand, associated also with its national rugby union team.  The image was designed by Reginald George James Berry, who designed the reverses for all coins introduced that year.
Answer: hole in the middle