5/32" is 3.969mm
1/8" is 3.175mm
In the UK O-1 precision ground stock is available in both metric and imperial thicknesses. I like the imperial sizes because 4mm always seems a little too thick, even after sanding out the surface grind marks, and 3mm feels a touch too thin unless you are doing a scandi-grind.
I have heard that both Ray and Alan considered that 1/8th would have been fine for the original Woodlore, but it was felt that the general public of the time (when they came out with the design) would want something a bit thicker.