Simply the date that I understand a particular stamp was first available to the public. This is nearly always the official issue date  as advised by the Post Office but there are a few items where there is a  well-established earlier date. Where all I have been able to find has been a  month then because my database program insists on a more accurate date I have guessed at the first of the month. Just don't ask me for a first day cover!

For the reprints in 2003 I have put the date sheets appear to have been printed which must be the earliest appearance but they may have been actually issued somewhat, even considerably, later. For the RMS issues I've used a similar approach.

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