Looks like it is a bed mat, but I don't know much more than that. In 1985 and later I see this:
112A15 MAT (UTILITY CARGO)-Serviced only in Kit, Group 642A94
And then when I look for that number I find this in both '85 and '86, as well as later. But nowhere do I find E5TZ 99112A15-B. It is apparently just part of that kit and wasn't sold separately.
Gary, AKA "Gary fellow":
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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