The military wiring harness produced on their M series vehicle (M series are the post 1948 vehicles built for military service to the common DOD and NATO standards) all had small metal bands with assigned wire numbers on them. The left rear service tail light on a Abrams tank would have the same number that the left rear service tail light had on an M35 or M38. Wire # 21.
You can see one of the small metal rings in this photo of a headlight ground wire.