CALLING all stamp collectors - note that a set of special stamps on the theme of the '1996 Olympic Games' will be issued on March 20. Four popular sports in Hong Kong are depicted on four stamps: gymnastics ($1.20), diving ($2.10), athletics ($2.60) and basketball ($5). Souvenir sheets with all four stamps will also be issued, but the colour used for the Royal Cypher, the Olympic rings and the inscription 'Hong Kong' will be different. The stamps are now on display at the General Post Office and the Tsim Sha Tsui, Tsuen Wan and Sha Tin Central post offices. Official first day covers are on sale at $1 each at all post offices. Advance orders for serviced first day covers will be accepted till Wednesday. The minimum number of serviced first day covers per order is five. An attractive presentation pack containing the four stamps will also be on sale at $19 each at all post offices from March 20.