7.2 inches base notch point spacing for Unertl, Lyman, Litchert, Fecker, El Monte and others with 50 clicks (25 marks) per knob revolution.
These old scopes have 40 tpi knob threads that moved the adjustment .025-inch per turn. Each click moves the scope .0005-inch, four clicks move it .002-inch. 7.2 inches divided by 3600 (number of inches in 100 yards) equals .002 inch adjustment change to move impact 1 inch at 100 yards.
It's these old external adjusted scopes that were the basis for one MOA equalling 1 inch at 100 yards. Not the trig value of 1.047-inch so many folks are infatuated with. Same thing for old iron sights based on a 30 inch sight radius which was average for 1903 Springfields with Lyman 48 sights on their receiver bridges. 30 divided by 3600 equals .008333-inch. Three times .008333 inch equals .025-inch. The pitch of 40 tpi lead screws used on those sights that had 3 MOA per knob revolution has been a standard for over 100 years.
