Hi Rich,
Isn't that where the thingamejig attaches to the watchamacallit ?.
After many TR's, I've never seen that fitting before....probably some PO's deft work. A good chance it's for something that couldn't go on the steering box side...but space is still pretty tight on the pass side too. Louder horns were popular to warn of approaching at speed. Perhaps an early radar detector. If it were local, maybe a "roo" scarer...sort of a whistle thing for clearing the wildlife.
All sorts of strange things were fitted over the years...we have a 3A that was tried by the Highway Patrol and it had a siren mounted behind the grille. That car's now been restored and the siren's back in the same drill holes, and all wired up to create havoc again.
Regards,
Viv.