Birthdate on tank
From Halopedia, the Halo wiki
The birthdate on a tank is a six digit serial number on the left front tread of the Scorpion Tank in Halo: Combat Evolved: 030569. This is a reference to Marcus R. Lehto, Halo's Art Director, who was born on March 5, 1969. On Halo: Combat Evolved for PC it is still present although it is less legible.