- 25p Euro 96 football stamp first day of issue on publicity leaflet.
I've just found something more recent, and that reminded me that I have some somewhere from 1967!
I'm excluding the USA from this thread because the USPS will cancel a stamp on almost anything - cola can, shoe, coconut, inflatable beach ball, if they can they will - on a hand-back basis usually, although some travel through the mail.
Some of mine went through the post before a philatelic handstamping service was available, but this is not one.
It is the 25p Bobby Moore stamp from the Euro 86 Football championship, applied to what looks like a souvenir programme but which is in fact part of Royal Mail's publicity, an 8-page leaflet. It has the Bobby Moore West Ham United handstamp.
What do you have?