Ah... is this for an external voltage regulator perhaps? My alternator has the VR on-board, so perhaps the bracket was simply a standard part of the firewall that went unused for the "frigid temps/high altitude" version of the truck with the 55 amp Alt and ammeter?
Yup... she's mine now! So interesting... I was researching where to put an on-board compressor and what that bracket on the PS firewall was for. From some other pics off BringATrailer.com, it looks like it is for a seatbelt detector?? So I was searching for that and up popped this thread on...