Take for example the Omega Speedmaster Professional 1970, also known as the first Moonwatch.
From 1969 to 1971, Omega equipped the Speedmaster Moonwatch with the original Moon bezel.
Looking at the bezel, a tachymeter (a ruler calculating speed as time passes in 100 m’), you can see a dot near each speed number.
At the number 90, the dot is above the right corner.
Later these bezels were called DON – “Dot Over Ninety (90)”. See the photo below.
After that (Mid 1971 onwards), Omega moved the dot next to the 90 (DNN – Dot Next to Ninety).