Complex sentence: The first stamp, the penny black, was put on sale on 1 May, to be valid as of 6 May 1840; two days later the two pence blue was introduced.
Split into simpler sentences: The first stamp, the penny black, became available for purchase 1 May 1840, to be valid as of 6 May 1840. Two days later, 8 May 1840, the two pence blue was introduced.