I don’t think the cap ratings are dual voltage ratings at all. They look like capacitance-voltage and this seemed to agree with measurements. For example in this supply, the 20(uF)-50(V) caps are half the volume of 40(uF)-50(V) - as expected. Back then, the capacitances were relatively tiny given the volume when compared to modern electrolytics. And there were plenty of “oddball” capacitances as well, like 30uF, 40uF, etc. Many different equipment brands of that vintage used those capacitors.
My understanding about the dual voltage markings is that the lower one is continuous. The higher one is inrush. That practice was inherited from the tube era -- at power up, the rectifies tubes with direct heat cathode warmed quicker that the indirectly heated amplifier tubes, so there was lower load and lower drop at first. The inrush condition settles in about 5 seconds or so and the voltages drop to running level.