And now we'd better backtrack a little, to deal with the Official overprints. In the earlier years, these consisted of the letters On S(tate) S(ervice).
From 1911 to 1930, the two types of overprint were used, differing only in the spacing:

Respectively, Types O1 and O2. Type O1 was used from the beginning; Type O2 was introduced in 1926. Type O1 is found on all three watermarks; Type O2 is supposedly only found on the third watermark. As usual with the Indian States, I'd suggest you take nothing on trust. If you have the time, energy and patience, I'd check the watermark on any Type O2 overprint. As I keep on saying, you never do know ...
Most of these stamps are fairly common, but there is the odd scarce combination of overprint and watermark. There are also endless numbers of overprint errors. Many are fairly obvious, such as double and inverted overprints.
Others are more subtle (and tend to be worth less). These are the inverted letters, 'O' and 'S'. When you think about it, it
is rather hard to pick an inverted 'O', and also an inverted 'S'. Never fear: help is at hand.
First an inverted 'O'

and an inverted right 'S'

The test is quite simple. The letters are dropped down below the line on which they should sit.
Many of these errors are quite cheap. The inverted 'O' above is listed at £4.50 and the inverted 'S' at £2.25. You won't make your fortune by digging through mountains of Travancore officials for them, but it
does relieve the tedium

In 1930, Travancore experimented with some new fonts for the overprint.

Type O3 - common on this stamp, rare on the 10 Cash pink

Type O4 - only found on this stamp, and common

Type O5 - common on this stamp, but quite nice on the 4 Chuckram green