context: In November 1944, after Greece was liberated from Germany, old drachmae were exchanged for new ones at the rate of 50,000,000,000 to 1. Only paper money was issued. The government issued notes of 1, 5, 10 and 20 drachmae, with the Bank of Greece issuing 50-, 100-, 500-, 1000-, 5000-, and 10,000-drachma notes. This drachma also suffered from high inflation. The government later issued 100-, 500-, and 1000-drachma notes, and the Bank of Greece issued 20,000-and 50,000-drachma notes. Third modern drachma coins The first issue of coins minted in 1954 consisted of holed aluminium 5-, 10- and 20-lepton pieces, with 50-lepton, 1-, 2-, 5- and 10-drachma pieces in cupro-nickel. A silver 20-drachma piece was issued in 1960, replacing the 20-drachma banknote, and also minted only in collector sets in 1965.  Coins in denominations from 50 lepta to 20 drachmae carried a portrait of Paul of Greece (1947–1964). New coins were introduced in 1966, ranging from 50 lepta to 10 drachmae, depicting Constantine II of Greece (1964–1974). A silver 30 drachma coin for the centennial of Greeces royal dynasty was minted in 1963.  The following year a non-circulating coin of this value was produced to commemorate the royal wedding.  The reverse of all coins was altered in 1971 to reflect the military junta which was in power from 1967 to 1974. This design included a soldier standing in front of the flames of the rising phoenix.
question: How many years after the first issue of coins was the silver 20-drachma piece issued?
answer: 6