Here's a further validation of Rick's method...
I have a set of Chapman gunsmith screwdrivers, individually sized bits that fit into a common handle. I bought an accompanying package that includes the inferial and metric hex driver bits (the type one would use on a SHCS).
Each of the inferial hex bits is stamped with a number, e.g., 8 on the 1/8 bit and 10 on the 5/32. For a long time I thought they were some sort of manufacturer part number.* Then one day (cue the head slap sound) it occurred to me...
8/64 = 1/8
10/64 = 5/32
A quick check of the other inferial bits proved that the stamped number on all of them is the number of 64ths corresponding to the size.
*In defense of my mental obtuseness, I may have been predisposed to think part numbers since the numbers stamped on the metric bits still make no sense to me as size indicators. The 4 mm is stamped "44" and the 5 mm is stamped "45" but the 2 mm is stamped "41" and the one stamped "40" measures 1.5 mm.