E2-BAG/1/7
From Halopedia, the Halo wiki
E2-BAG/1/7 is a United Nations Space Command infantry unit
It is of unknown type or size, although it's presumed to be a Marine unit.
It is possible the E stands for the company, Echo. The 2 may be second platoon of E Co, or second battalion, which the company is part of. BAG could stand for Brigade (or Battalion) Assault Group, which would be the infantry element of a reinforced Marine regiment or battalion, or Marine expeditionary unit. The 1/7 could either be 1st Battalion, 7th Marine Regiment, or 1st Regiment, 7th Division. This is simply conjecture and should not be taken as canon.
E2-BAG/1/7 was interviewed by Dr. Robert McLees on multiple UNSC and Covenant vehicles and firearms that will appear in Halo 3. Some of these interviews were posted on Bungie.net's front page.
