OK, here goes.
I took your original image of the rear of the stamp.
I then converted it to black & white and got ride of the excess so it's a clean canvas to look at.
I then flipped it horizontally, as if you were looking at it from the front of the stamp.
For it to start with a "C" it had to be inverted, so I then flipped it vertically, as if it had been applied upside-down.
It looked like the three stray perfins were the result of it being offset to one side when applied, so I moved those to approximately where they would be on the opposite side if there was no offset.
Looks like "CNR" (BNAPS 24).