When open, the cut out square on the base is marked along all four sides in Imperial or Metric scales. (It is still a surprise, or is it, that we sell ten times as many Imperial models than Metric)! This enables the number of threads and weave to be counted to the inch (or centimetre) and therefore to determine the quality and strength of the material.
Black Japanned mount with square aperture. Double lens system reduces any distortion at the edge of the field.