The 1 Anna Service (= government use) Indore State stamp on this cover


(SG S3, sent from Kanawad on the 16 September 1906, arriving at Indore on the 19 September)
is common, both mint and used - but not so still on cover.
It was issued in 1904, when the Indore post office was open to the general public of the State, and there was some point in distinguishing government mail.
In 1908, the Imperial Indian Post Office forced a merger with the Indore State Post Office; henceforth, Indore stamps were only to be used for Indore government mails. The public had to use the Indian Post Office.
As there was no longer any need for these Service overprints, the remaining unused stocks were dumped (possibly at less than face) on the European market. Hence the Gibbons valuation of 25p for the 1 Anna stamp mint.
There seems to have been a strong realisation of the value of used Indore State stamps in Indore at the time, too, because the used stamps are also easy to find; Gibbons rates them at 25p each too.
But finding any of these SERVICE overprints on cover, used between 1904 and 1908, is another matter. The waste paper merchants of Indore must have done a very nice trade indeed in mutilated covers from government departments, something over a century ago.