I don't know specifically about the '41 but the early AD trucks didn't use a relay so I'd guess the wiring diagram you have is right.
With that set-up the wire through the steering column was hot. If it wears through the insulation you have troubles.
If you convert to relay one terminal is hot, one goes to horn and the third is the ground, which will run up your steering column. If the insulation wears through on it all it does is honk your horn.
