Interesting. Certainly not Mustang. The part number is a TYPE 9 Transfer (tailgate), and specifically:
Chamois (1981/85) E1TZ 9942528-AT (Not replaced 1/94)
As for the instructions, here they are and I've saved them to be put on the tape stripe page, w/a reminder in my ever-growing spreadsheet.
Gary, AKA "Gary fellow":
Profile
Dad's: '81 F150 Ranger XLT 4x4: Down for restomod: Full-roller "stroked 351M" w/Trick Flow heads & intake, EEC-V SEFI/E4OD/3.50 gears w/Kevlar clutches
Blue: 2015 F150 Platinum 4x4 SuperCrew wearing Blue Jeans & sporting a 3.5L EB & Max Tow
Big Blue: 1985 F250HD 4x4: 460/ZF5/3.55's, D60 w/Ox locker & 10.25 Sterling/Trutrac, Blue Top & Borgeson, & EEC-V MAF/SEFI