Here is an example of a revenue stamp used for postal purposes, from the Indian State of Kishangarh:

In 1948, just after India gained independence, the formerly separate princely States of Rajasthan were merged into the new state of Rajasthan. Some, including Kishangarh, had operated their own post offices before the merger.
During the transitional period from 1948 to 1950, Kishangarh overprinted just about every postage stamp it could find 'Rajasthan' in Hindi. It still ran short of a few values, and so authorised the use of the blue 1 Anna stamp, which was normally a revenue stamp, for postage for a short while. Usages like this are quite scarce, and worth far more than the ordinary - revenue - usage (and only to be bought with a cast-iron certificate of authenticity, which this cover has).