I am shocked! Shocked, I tell you...
No body has mentioned that the vintage Sansui on/off power switch is part of the speaker selection knob. The knob positions are:
power off
power on with speakers "A" selected
power on with "B" selected
power on with "A" and "B" both selected
power on with neither "A" nor "B" selected
Connecting the speakers on terminal "B" with "A" unoccupied allows one to power up the amp and let it settle in a few minutes before switching to the "B" position to connect the speakers.
The relay that waits a few seconds at power on basically does this for you, but maybe not as long as one would prefer.
Same process in reverse when powering off - with the speakers on "B", the power off is done by turning through the unoccupied "A" terminal without "B" connected, so the actual power off includes only a power off event to anything on "A", not "B"... in effect the "B" terminal is disconnected before power off.
I don't know the actual design/operation philosophy of combining the power and speaker selections this way, but it seems to offer some apparent possible fail-safe function depending on how you connect the speakers.