BBBS, The grips on your Model 18 are target grips and will not be stamped with the serial number. Normally only service or Magna grips are numbered because they were sanded mounted to the gun frame before finishing to match the contour of the frame. Target grips cover most of the frame so do not require sanding to the frame. The grips may well be the proper grips and certainly appear to be of the same era. Just as a note, the pencil marking of the grips is a pre-war method, post-war grips were normally stamped. Hope this helps. John.