J81 is the small Duck-Tail trunk-lid spoiler that sits across the back edge of the deck lid. About four inches tall, body-color, integral to the trunk lid contour. On AAR 'Cuda + T/A Challenger the spoiler is a standard part of the package (always present alongside A53). On other E-body trims it was an optional dress-up.
Distinct from the much taller wing on Chargers Daytona / Superbird (which is a structural fixture, not optional) — those cars use a different package entirely. J81 on a Charger Daytona or Superbird tag is a misread or swapped tag.
The 1970 Hemi 'Cuda carries J81 routinely (paired with J78 Sport Slats — the rear-window louvers). Together they're the visual signature of the 1970 'Cuda muscle pack.