This advert is sorta correct. Ford used the same letters, as shown below, on the 1978 - 81 pickups, but not out to 1983 as it shows.
Note that these are D9TZ's, meaning they came out in '79. More specifically they came out in August of '79. Prior to that the letters were D5UZ's, but the D9's replaced them in 8/79.
16606 LETTER - Incl. attaching parts - Also refer to groups 8316 & 40282
80/ E 100/350, F100/350, U150
D9TZ 16606-A "F"-hood
D9TZ 16606-B "O"
D9TZ 16606-C "R"
D9TZ 16606-D "D"
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