5bc3 why such a weird tube

[Tubejunke]

I have a 58 Zenith Space Command that uses a 5AU4 instead of the pretty much standard at the time 5U4. I may be wrong, but it seems like the specs were almost identical and I can sub the 5U4 in with no problems. This particular chassis design was used for a number of years and through the run they used both tubes. A power supply/rectifier is a pretty basic and straight forward circuit that you wouldn't think would require much design change.