You'll find most battalion and above units in the Army will have a HQ50 vehicle. That alone will tell you nothing about the history of the truck. HQ50 only means that it was #50 in the headquarters order of march, even then vehicles are not numbered consecutively. You'll find most correspond to the section or position of the person it's assigned to. E.g. 6 belongs to the commander, 5 belongs to the executive officer, 7 belongs to the top enlisted advisor, 3 belongs to the S3, 4 belongs to the S4 or supply sergeant, etc. So a vehicle marked HQ40 would be part of the BN/BDE/DIV S4 or G4 section, or HQ37 would be the S3 sergeant major. The executive officer doesn't have a section, so 50 wouldn't work like that. Sometimes, the number means nothing at all and is only assigned as an unique administrative identifier to differentiate from other trucks in the headquarters. Bottom line, your truck was a support vehicle for a battalion, brigade, or division staff. Uncover the markings on the left side and we can tell you more.