I didn't realise that I only had one article on pentagonal stamps so far.
So, time for a small update...
In 2000, US post emitted a sheet with 5 pentagonal stamps dedicated to the exporation of the solar system.
The sheet indeed shows us several planets, with Saturn and a/the Moon as background and 5 pentagonal stamps forming a bigger pentagon.
|full sheet 'exploring the solar system' (USA - 2000)|
3 of the 5 stamps are pictures of the Sun, it's centre and the eruptions on the sun's surface.
The other 2 stamps are depicting the Earth, seen from space and the sky, seen from Earth.
|detail with 5 pentagonal stamps|
The whole images of 5 stamps, forms another, bigger pentagon again.
|detail of the back side of the sheet|