Did there used to be an Army regulation or pamphlet that caused the commanding officer's jeep or vehicle to be numbered with a -6 suffix on the bumper, as in HQ-6, A-6, B-6, etc? During my time in service there was no such requirement that I'm aware of, but the tradition was still put into practice. If I recall correctly from my military history reading the -6 suffix also was sometimes used informally in radio call signs until Signal operating instructions got away from that procedure.