Those markings are meant to be at 1/10mm. 4/10 mm, 5/10 mm etc. Round numbers, multiply by 4 and divide by 100. 4/10mm = .016” (mod), 5/10 mm= .020” (1/2) etc. There are 25.4 mm/ inch, so this isn’t exact and measuring with a bore gauge will correct any manufacturing tolerance issues.

Greg