Gunman;
I believe you have your decimal point one place off. 1 mm = .03937" (1/25.4) or approximately .040". Thus .1 mm = .004" while .01 mm = .0004" or a bit less than thousandths.
I was thinking the sizes were only stamped to one decimal place but wasn't absolutely certain so didn't say it that way.
In machine shop lingo, although not perhaps technically correct .1 was called 100 thousandths, .01 10 thousandths, .001 one thousandth &.0001 one tenth, which meant it was a tenth of a thousandth. This was universal among all machinists I ever knew & many of them came from widely different locations.
