Upon implementation of the CARS Reorganization in 1959, Regimental Headquarters of each of the regiments selected to be parent units under CARS were inactivated and transferred to Department of the Army control. This effectively made the Secretary of the Army custodian of those colors, which are permanently displayed in or near his office in the Pentagon. (This partly answers Mary Ellison's question in her recent posting, #15569 of 19 August - which I'm working on now.)
Under CARS, very simply, the battalion replaced the regiment as the principal tactical maneuver element. That is, for all practical purposes, the battalion became autonomous. Battalions have since carried regimental designations but this is mainly, if not solely, for the purpose of perpetuating the lineage and honors of the "parent" regiment.
Thus, when an armor battalion is inactivated, the battalion standard - not the regimental - is returned to the issuing authority, The Department of the Army, which orders it to be placed on public display at The Armor Center, Fort Knox, Kentucky.
Also, in my first reply, although irrelevant to your inquiry, I neglected to mention the battalion was also assigned to 25th Infantry Division during its Vietnam service.