Initially, they stamped holes into a letter then used sulfur gas to sterilise it, making the letters appear yellowish. 'The gas penetrates the envelope through the holes,' he said.

The doctor in his late 40s is a member of the Disinfected Mail Study Circle, a group founded in 1973 to research the practice of disinfection and quarantine treatment of mail.

'I could be the only one collecting [the disinfected letters],' he says, and smiles as he carefully takes out the letters from their protective cases.

Many of the letters are from Italy, which is evident from the Italian script written on the envelopes. Pointing at a word on an envelope, the doctor who knows more French than Italian said: 'That was like today's Hospital Authority [in Hong Kong] during that period of time.'

The doctor can identify the various impressions on the envelope that were made by different tools: a chisel or a restel. 'A restel works like a waffle machine,' he says. A pile of letters were placed between the jaws of the machine and they were stamped through, he said.

'In Parma [in Italy] they stamped five holes into the letters,' he said, adding that it was a unique pattern for the city. These small holes show how authorities reacted to big epidemics. 'In 1883, people knew the spread of cholera was unrelated to letters. Then they cancelled the sterilisation.'

The practice was shortly revived during the second world war in Europe and when smallpox hit Hawaii in 1889. 'In 1883 there was an epidemic in China. Some 20 to 30 letters were sent overseas during that period of time, and I've got three to four of them.'

Dr Cheung is also chairman of the Hong Kong Philatelic Society and he only became a serious stamp collector after a 1971 exhibition. The doctor now has a collection of more than 10,000 and he describes his three decades of stamp collecting as a lot of 'blood, sweat and tears'.

'Blood, because I spent so much money. Tears, because sometimes I can't get the stamps I want. Sweat, because the search [for that one stamp to complete a collection] can be so difficult.'

And his passion for collecting stamps is visible at his workplace, a family doctor's clinic in Sheung Wan, where one sees many clues.

The most obvious one is the red letterbox once used during Hong Kong's colonial era, which sits in the patients' waiting area and a special room has been dedicated to stamps.

Some have his face printed on them and there are special edition stamps from various international exhibitions on display on every wall.

The pamphlets piled up in the corner are a sign of the doctor's connection with the annual stamp fairs in Hong Kong. He often organises the event.

Completing collections of stamps spurs his drive to keep collecting. 'A stamp collection can be completed and that's what gives you satisfaction. When you are one stamp short and go find it, you're given a purpose.'

It is not about money or investment, as one can hardly predict the future price of a stamp, he says. 'A Tsing Ma Bridge souvenir sheet, which many people bought, has dropped to one tenth of its value.'

However, a stamp released in the Monkey Lunar New Year of 1980 is priced at around HK$3,000. Another stamp featuring Peking Opera artist Mei Lanfang published in 1962, featured in an auction house's leaflet, rose to Euro2,000 (HK$21,800).