It is 3rd party, not Ford, but could be Dealer installed.
Trailer brake wouldn't make sense since a handle like that would be hooked to a cable and trailer brakes can't be cable hook ups.
Was there a 5th wheel hitch in the bed, or a hard tonneau cover, could be a cable release for something like that.
Could even be a Neutral shifter for towing 4x4 behind motor home.
I would find the cable coming out of firewall and see where it goes