Here are where the numbers are in the skull. From left to right the way a clock goes round:
the 1st number 3 is the eyeball on the left
The number 2 is just above the right eye
The last number 3 is lower, on the right corner of the where the skulls mouth would be, along the edge of the face.

I have looked for other numbers in this card but these are the only fully created numbers, the others squiggles and marks are only half made numbers. Therefore, I belive the clear numbers hidden in this card are 2/23.