unknown 1950 1952 1955 1958 1959 1960 1961 1963 1964 1965 1966 1967 1969 1971 1972 1973 1974 1975 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991