Postmarks on the banknotes

Sometimes the postmarks, which are expected to be on a envelopes can be seen on banknotes.

Appearance in some cases are not clear. For example the Ceylon banknotes can show a postmark from time to time. Here it can be observed on a less common issue of George V  on its reverse side:

https://www.delcampe.net/en_GB/collectables/coins-banknotes/banknotes/sri-lanka/great-britain-1917-1939-ceylon-1-rupee-1918-rare-low-start-no-reserve-1581667450.html .

This banknote 1 rupee bears two dates: the date of issue 23 March 1918 and the date on the postmark 21 July 1921.

In this particular case the reason for double date is not clear. Though there are other money which bear postmarks for obvious reasons.