Smauggie, that was the official seal. It was applied to all postage stamps before issue. Here is a sheet of SG 28 (the 1912 ½ Anna), showing the seal:

It was also applied to all postal stationery,
below the indicium for some reason, as you can see on the postcard. Here is a cutout from a ½ Anna postal stationery envelope

showing the same thing (and which I should post in Rod's Mutilated thread).
Occasional stamps escaped without the seal, and are worth big bucks. I still remember the palpitations that cutout caused when I first set eyes on it. There
is a ½ Anna green stamp, SG 30a of 1916, known without the seal, and catalogued at £1400 used but unpriced mint. It's printed on wove paper, whereas the envelope used laid paper. It only took a few moments to work out what I had.