| Letter | From | To |
| V | August 1979 | July 1980 |
| W | August 1980 | July 1981 |
| X | August 1981 | July 1982 |
| Y | August 1982 | July 1983 |
| A | August 1983 | July 1984 |
| B | August 1984 | July 1985 |
| C | August 1985 | July 1986 |
| D | August 1986 | July 1987 |
| E | August 1987 | July 1988 |
| F | August 1988 | July 1989 |
| G | August 1989 | July 1990 |
| H | August 1990 | July 1991 |
| J | August 1991 | July 1992 |
| K | August 1992 | July 1993 |
| L | August 1993 | July 1994 |
| M | August 1994 | July 1995 |
| N | August 1995 | July 1996 |
| P | August 1996 | July 1997 |
| R | August 1997 | July 1998 |
| s | August 1998 | February 1999 |
| T | March 1999 | August 1999 |
| V | September 1999 | February 2000 |
| W | March 2000 | August 2000 |
| X | September 2000 | February 2001 |
| Y | March 2001 | August 2001 |
|
| Year | March | September |
| 2001 | - | 51 |
| 2002 | 02 | 52 |
| 2003 | 03 | 53 |
| 2004 | 04 | 54 |
| 2005 | 05 | 55 |
| 2006 | 06 | 56 |
|