Jim Carter sells the straight bracket, and has this note:
Secures the original horn to the intake manifold. Is correct for the 1937-1952 Chevrolet and 1939-1952 GMC. Not correct if you use a manifold secured oil filter.
Jim Carter Trivia: The horn and oil filter can not fit in the same space. Thus this comfirmation requires a different design horn bracket. ( Once included in with the dealer installed oilr filter assembky)
The bold text was me, the typos are Jim's.
That could shed some light on why there are a couple different types.
That got me thinking that I may be mistaken about which one I have, because even though my truck had a remote oil filter, it was mounted on the inner fender rather on the manifold in front of the carb. I just went on a quick search to confirm, but couldn't find my horn and bracket. It got repainted a while back and stored away in the trailer and it's below zero right now, so didn't spend much time digging.