Interesting point, Rodney: I hadn't noticed it.
On the other hand, I have a pitifully small collection of covers from Travancore and Travancore-Cochin. The impression could be affected by the fact that most of the surviving covers are registered. The only un-registered T-C cover I have is this one:

It was sent from (somewhere) in the old Travancore State to Trichur in the old Cochin State, and has only a Trichur receiver - but it would be risky to generalise from a sample of one.