Since NTP poker cards have white edge around the back, the invisible marks can also be marked in the white edge as well as in the middle. Our technicians can mark the 4 white corners with tiny marks, including the numbers and suits. Though the tiny marks at white corners are not as big as those big marks in the middle, they are very clear to see in short distance for the clear black marks in the white area. Usually when it's difficult to make clear marks in the middle for special patterns of the back, people may choose to make marked cards with tiny marks in the white corners.
As that shows in the picture, it's Spade A in the left card and Heart K in the right card.