Indian States / Travancore Has Me Scratching My Head

Trying to id this official overprint. Is it O1,2 ,3....8? The watermark is C, The design is 1 according to Gibbons. But I can't tell which overprint this is?
Thanks

I am not understanding you.
Travancore : Kingdom

Is the Watermark Upright or sideways?
What is Watermark C?

It is Stanley Gibbons O10 4 Chuckram slate green, Opt Type O1
If the OS is 13mm wide it is O1
If the OS is 16.5mm wide it is O2
Looks like a "Rounded O"

Best I can do

are you sure it is a "C" wmk?
Gibbons lists Upright "C" for 023-027 but there is no 4 chuckrams with this overprint.

As already stated above, this is SG overprint O1, but a 4 anna watermark C does not exist with this overprint. Can you post a scan showing the watermark please?

Travancore is notorious for difficulty in reading watermarks.

That being said ,if it does in fact have a "C" wmk;it is a unlisted variety.
When I go through lots with these stamps;they all go in wmk fluid.Very time consuming.Upright,Inverted,horizontal;all has to be checkrd.

Thanks for all the help. I got deep into the weeds trying to figure this one out along with two other of this type and about half a dozen other feudal state stamps. It was late. I've never had a close look at these before. I tried for the first time to make a scan of a watermark, using my flatbed, lighter fluid and a bottle of pelikan black fountain pen ink laid on top of the stamp. It came out better than I hoped. AND showed me that i'd mis id'd the wmk. It looks more like B. The overprint is 13mm wide so O10.
Thanks
Greg

Yes, definitely watermark B.