TO help celebrate yesterday's launch of the Bank of China banknotes, Hong Kong's General Post Office will launch a special postmark and the bank will bring out a set of commemorative souvenir covers. However, both the postmark and the souvenir covers will only be available today. An intricate spiralling pattern has been used as the design of the postmark, which will be used for the cancellation of souvenir covers privately made by philatelists at the post office's six philatelic centres. The postmark has been designed by Kan Tai-keung. The souvenir covers issued by the Bank of China will be put on sale by bank staff at the public hall of the General Post Office today. No commemorative stamp will be launched for public sale to mark the occasion, but a stamp has been specially prepared for the souvenir covers. The $1.40 special issue stamp of ''Hong Kong Building of the Future'' has been designed by Ian Leung. The stamp's picture depicts the Bank of China building in Hong Kong and will be put on the envelopes used for the souvenir covers. The post office agreed to introduce the postmark because it considered the theme to be appealing to collectors and the occasion historically significant. Strict rules surround the introduction of special postmarks.