In Coxeter notation can be represented as [6,4*], removing two of three mirrors (passing through the square center) in the [6,4] symmetry.
Escher explored the concept of representing infinity on a two-dimensional plane.
Escher's wood engravings Circle Limit I–IV demonstrate this concept.
The last one Circle Limit IV (Heaven and Hell), (1960) tiles repeating angels and devils by (*3333) symmetry on a hyperbolic plane in a Poincaré disk projection.
Without this variation, the art would have a 4-fold gyration point at the center of each square, giving (4*3), [6,4+] symmetry.