Btw, before WW2; many Sonor drums came with square rods. The company did not go all slotted for all models until the mid 50s at the start of the Link Era which is why the slotted rods are a hallmark of those decades…. In my opinion.
So Sonor actually went back to something they had done pre war by changing over from slotted to square.
Here is the Model USA from the 20s that featured square rods.
