View Here : Tyre Width Vs Rim Width
In 1938, wider rims were used for better tire performance. With wider rims the tire beads were not pulled in as far, which allowed the tread to flatten and with a deeper tread the tires became more efficient. With the wider rim the same size tire has a wider tread width. The second sizes (II) still used the approximate tread width.
I think the old rule of thumb was one tire size up or down on a rim is ok. Not ideal but ok. Firestone is correct the the corresponding rim width for a 12.4 tire (it's 12.4 not 12. is 11". You could get by with a 10" or 12" rim but I'd say a 9" rim is a bit of a stretch - so to speak.