The first picture looks to have E4DB which would mean it was originally designed
for a 84 LTD, the small LTD that was foxbody based. Its a clean part so went
somewhere inside the car. The E4DB is the prefix of the part number, following
it is the basic part number that would be something like 6123106, this number
would tell what it is, where it goes but you say its not there. Maybe behind the
kickpanel is a good guess since that area is more likely to be the same on a Mustang
as a foxbody LTD rather than the rest of the car.