At the end of the "Rubble and Ruin" level, Fairburne addresses the ranger as "lieutenant" whereas the rank patch on the sleeve of his uniform is that of a "technical sergeant" (pay grade 2) (3 up and 2 down).
In the Secret Weapons level there are Vietnam War-era U.S. Army ALICE packs in the armory.
At the end of "Rubble and Ruin", the technical sergeant that Fairburne addresses as "Lieutenant" is wearing his rank insignia on only only one sleeve and he's wearing a Ranger patch in place of a division insignia.