just rebuilt my 4-speed and had a broken shift fork that I replaced with a steel one, just for my information, Why were they made from Bronze from the Factory? anyone know?

I really don't know why but going back to my metalurgy classes, bronze was preferred over steel because they can withstand sudden high impacts better. Think about swords.
Less brittle than iron and cheaper than steel.
Typical cost-effective compromise?
"Banging " through the gears?

I believe it's because brass is sacrificial, it wears out instead of the synchro sleeves.

'71 and later forks are steel.