They represent thickness as measured in thousandths of an inch such as 35/1000″, 50/1000″, etc… This is the form of measurement that our machine shop is conversant in and as such became our primary unit of measure for the picks.

To translate to millimeters the 40 would be 1.0 mm, a 50 would be 1.25 mm, and a 60 would be
1.50 mm.