Just to help a tiny bit, since Heller is (was?) a French company, the meaurements are in millimeters. And I do not know what .9 mm translates to in inches. It seems that my Math degree has expired after 40 years hanging in the back of the closet with my naval officer's sword. I never used either one of them.
I am ordering more scratch paper to work it out, but the general idea is that there are 25.4 mms in an inch, so .9 of one of those mms is going to be somewhere slightly greater than 1/32".