I found that this strap should be on A bodies and E bodies that loops around the positive battery cable and the shock threads on the inner fender well drivers side. I looked at various survivor photos and some factory photos from back in the day and some cars had them and some not. I am asking specifically for 1971 Duster. Anyone know of these?

Yeah, likely to prevent cable from chafing on shock stud, and quicker than feeding cable through apron harness retainers.
Some things got missed on occasion on the assembly line, or misplaced over 50 years.
FSM or engineering info may refer to it in an illustration.