A letter bearing the Penny Black stamp of 1840, used five days before it was legal to do so, is a highlight of Stampa '98 at the RDS Dublin this weekend. It is expected to fetch £200,000 when it is auctioned next year. Thousands of stamps are on show at the exhibition which ends tomorrow. The rarest Irish item is a strip of four twopenny stamps issued in 1935 for vending machines and now valued at £50,000.