March 1950 – April 1951  :  
April 1951 – October 1951  :  
October 1951 – March 1952  :  
March 1952 – February 1953  :  
February 1953 – April 1954  :  
April 1954 – August 1954  :  
August 1954 – May 1951  :  
May 1955 – September 1956  :  
September 1956 – October 1957  :  
October 1957 – September 1958  :  
September 1958 – July 1959  :  
July 1959 – August 1960  :  
August 1960 – September 1961  :  
September 1961 – August 1962  :  
August 1962 – August 1963  :  
August 1963 – September 1964  :  
September 1964 – February 1966  :  
February 1966 – May 1967  :  
May 1967 – September 1968  :  
September 1968 – September 1969  :  
September 1969 – June 1970  :  
June 1970 – July 1971  :  
July 1971 – March 1972  :  
March 1972 – September 1972  :  
September 1972 – January 1974  :  
January 1974 – February 1975  :  
February 1975 – July 1976  :  
July 1976 – August 1976  :  
August 1976 – September 1976  :  
September 1976 – July 1978  :  
July 1978 – August 1979  :  
August 1979 – April 1981  :  
April 1981 – October 1982  :  
October 1982 – June 1984  :  
June 1984 – February 1986  :  
February 1986 – December 1987  :  
December 1987 – February 1989  :  
February 1989 – June 1990  :  
June 1990 – April 1992  :  
April 1992 – March 1994  :  
March 1994 – July 1995  :  
July 1995 – November 1996  :  
November 1996 – June 1998  :  
June 1998 – July 1999  :  
July 1999 – July 2000  :  
July 2000 – April 2002  :  
April 2002 – June 2004  :  
June 2004 – September 2005  :  
September 2005 - November 2005  :  
November 2005 - September 2007  :  
September 2007- September 2008  :  
September 2008 - September 2009  :  
September 2009 - May 2011  :  
May 2011 - April 2012  :  
April 2012 - October 2012  :  
October 2012 - present  :