I read it that the cestus DX penalty, when only one is used, is to be exactly the same as a main-gauche. Let me explain why.
First, ITL page 113 is states that "A
single cestus is treated exactly like a main-gauche". So it is clear that it is like a MG only when it is a single cestus.
Then when you read about two cesti in the next paragraph, it is the DX penalty that is treated differently.
I would also add that a cestus is not a parrying weapon. A main gauche or a sai has quillions meant for catching the blow. A cestus is basically a brutal boxing glove. Here is a roman sculpture of a cesti boxer "Boxer at Rest":
https://en.wikipedia.org/wiki/Boxer_at_Rest
As you can see, nothing about that makes it a weapon that would excel at parries.
BTW, if you want a good thread about main-gauche and two weapons seeL
http://forums.sjgames.com/showthread.php?t=168333