That's the style of 50 years ago. High amperage circuit breakers really went at a premium so the engineers came up with the 6 circuit rule and the split bus.
If top fed, the largest loads would be ganged there -- a five double-pole maximum -- with the sixth position leading off to the daughter bus directly below.
The daughter bus could have far more breakers -- overwhelmingly 15A single poles.
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One must be on the look-out for bottom fed split bus panels. Obviously, the scheme is turned upside-down.
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Beyond that, the panel designs from the 1950s are truly scary.
Very tight quarters for our field conductors.
Lots of live exposed bussing -- typically two straight copper rails. This layout makes 240V circuits or running 14-3 very odd. To get to the other phase one must cross over to the other side of the breaker array! It also means that you don't have a chance in hell of working a two-pole handle tie!
When the budget permits these panels need to be retired.
In the mean time work on them cold and meg them before power-up.
