Who has clear evidence of the early usage of any classic issue?
My passion for Australian Postage Dues and my general approach that if you have ten copies of a stamp, you may as well have eleven, or twenty, or two thousand, has led me to the following stamps:

The first series of Postage Dues (and some values of the second series) were distributed from the printer in Sydney on 24 June 1902 to Queensland, South Australia, Tasmania and Western Australia. Note that the first issue was not sent to New South Wales and that none of the monochrome (green) Australian Postage Dues were validly used in Victoria.
The earliest known date is a 2d stamp used in Adelaide postmarked 1 July, 1902 with all values of the first series in use by the end of July, 1902. The first stamp in the scan is the earliest date I have, Jy 11 02 in Adelaide, just ten days after the earliest known. The second and third stamps show consecutive dates in August 1902. The 13th in Port Victoria, South Australia and the 14th in Hobart, Tasmania.
What early dates can you show?
PS: I can remember being offered a first day usage of the 1d red Aussie Kangaroo or a stockbok chock full of toned used Aussie Postage Dues for about $600 in the late 1980s. I chose the dues, of course but the FDC has increased a little more in value than the choice I made.