After knowing that North American F40T12 U bend tubes were both made with 6 inch and 3 inch leg spacings being known as U6 and U3 to denote their respective leg spacings, I have known that there were F32T8 U bend tubes that were meant to replace the F40T12/U6 U bend tubes and NOT the F40T12/U3 U bend tubes. However, the F40T12/U3 U bend tubes were instead replaced with F31T8 U bend tubes with an even smaller 1.625 inch leg spacing instead.

I would really like to know why North America never opted to design a F32T8 U bend tube with a 3 inch leg spacing to replace the F40T12/U3 U bend tubes and instead opted to design the 1.625 inch leg spacing F31T8 U bend tubes, which would require a completely different fixture than the F40T12/U3 tubes.