These markings, a circle with a cross on the inside and a square with a cross in it were stamped into the sheetmetal access hatch . The stamping was convex on the outer portion of the hatch and most boats had this hatch with the symbols painted the same gray as the rest of the conning tower ,but.......... on some early war boats had these raised symbols painted in black ( for a great color view please see the front cover of U-BOATS by ANTONY PRESTON ) . I do have other color shots of this ,but as the war dragged on this appears to have been dropped in order to save time ,picking out the symbols was not really practical anyway as they were already raised and visible.