The new jumbo stamp features the 1888-9, New South Wales Centennial issue (SC #86, New South Wales). The 20 shilling, Centennial stamp is depicted in multiple format to represent the full sheet of 50 held in the National Philatelic Collection. The sheet is believed to be unique.
The "specimen" overprint was applied to archival sheets to prevent possible misuse of the stamps for postage.
According to Gibbons Monthly, the original stamp is widely regarded as the world's first commemorative since it bears the inscription "100 Years". It was also the first Australian stamp to depict a living person other than Queen Victoria.
To visit the Australian National Philatelic Collection, click here.