Now I know you're all positively champing at the bit to get on to the Maharaja's 27th Birthday set, but I'm afraid I'll have to drag you back to the official overprints for a while.
In 1932, there must have been a shortage of certain values, because the authorities seem to have called in all stocks of the 5 Cash, 10 Cash and 1¼ Chuckram official stamps, and surcharged them 6 Cash, 12 Cash and 1 Chuckram 8 Cash:

Just about all the flavours of On S S overprint were represented, with all three watermarks turning up somewhere in the mix. Some of the combinations are distinctly scarce: the 6 Cash surcharge on the 5 Cash with the first On S S overprint on first watermark paper, for example. (There can't have been many of them left to surcharge.) You can also find all the inverted letters etc as well:

SG O75e - inverted left S
BTW, don't think that these inverted letters must be frightfully expensive, and only for collectors with far more money than sense. SG O75e there catalogues at a modest £4.75, and would sell (if it
was for sale) for a good deal less.
In 1939, in the last days of the old Conch stamps, Travancore finally did away with the On S S overprints, and introduced a single line SERVICE overprint. This was printed from loose type, so that the letters aren't always regular. This overprint was applied to the 6 Cash and ¾ Chuckram stamps only:

In 1940, there was a small run of the ¾ Chuckram and 1½ Chuckram Conch stamps with a new SERVICE overprint, printed from plates:

The two are fairly easy to tell apart. Focus on the R. In the loose type overprints, the tail of the R descends vertically; in the plate prints, the tail of the R descends at a roughly 45° angle.
As the new perforators had just been introduced at the same time, it's possible to find a variety of perforations on these stamps: 11, 12, 12½ and compounds of these. In this block, the top row of perforations gauges 12, and all the rest of the perforations are 11, making the top three compound perf 11 and 12 (SG O94d) and the bottom three perf 11 (SG O94b):
