This stamp is a Michel 8, either 8a or 8b (8F is the inverted color and 8c is printed on plain thin paper.) You can inspect genuine examples here. It is an example of the thick quadrilled paper used in Michel 6yg, 6yh, 7yc, 7yd, 8a, 8F, 8b, 9 and 10. It is called gegittertes in Germany (or more completely, dickes, gegittertes Papier — dickes meaning thick). The paper pulp is poured onto a fine wire mesh and when it dries, a wire mesh impression is left in the paper. The pattern could also be called a grid, a grill or a lattice. The mesh is square to the edges of the paper, parallel vertically and horizontally; IT IS NEVER A FAINT DIAGONAL OR DIAMOND PATTERN AS SUGGESTED BY No. 1 BELOW. Note that Michel 17a and b also have a right-angled quadrille pattern, but it is very hard to discern. So supposedly do Mi 18a and 18c, but I take that on faith at this point. Lastly, Mi 18b is supposed to have a "waffle" (diamond) pattern.
Here are seven purported Michel 8's seen first from the front (somewhat enlarged) and then from the backside (more enlarged) Only one is genuine.
