I was moved to start this little thread by a current Spink auction catalogue, where this stamp is wrongly identified.
First off, I'm sorry for users of Scott, but apparently Scott doesn't distinguish this stamp. Gibbons
does: it's the 2 Anna Devi Singh type rose-carmine in Gibbons' 'wide' setting. The difference between SG 35A and 35aB is worthwhile: SG 35A is priced at £4.25 mint and SG 35aB at £600 mint.
The differences between the 'narrow' and 'wide' settings are subtle in these Devi Singh type stamps, but they are very important. The 2 Anna, SG 35aB, came from one printing only - Printing 6 - which was probably made in 1945/1946.
Printing 6 produced characteristically
tall-looking stamps, with long top and bottom margins and short left and right margins. Here is a genuine SG 35aB:

and here it is again, alongside a 2 Anna from Printing 5, with which it's most often confused:

So why is it so expensive? The print run was very small: only 400 stamps. As you can see, the shade was pretty close to that of the previous printing; many collectors probably decided it wasn't worth bothering about. Then, it appeared in the rather chaotic period between the end of WWII and Indian Independence in 1947.
Finally, there was absolutely minimal need for a 2 Anna value. It would have paid the double weight rate for letters, if the citizens of Barwani had been in the habit of sending overweight letters. (But they don't seem to have been.) The one use found for the 2 Anna value, and quite a common use it was too, was in pairs to make up the 4 Anna registered letter rate:

So it seems pretty likely that the majority of the 400 copies were used, and then lost to collecting, on registered letters.
If you believe you've hit the little jackpot and found an SG 35aB,
do post it up here first. It may save you some embarrassment later.