My AT-tyle PSU has a little lead coming off one of the molex connectors, a black and red lead with a little 2-pin socket like you might attach to a motherboard header.

Reading this post I guess it was for a fan, which presumably dates the PSU to the early-pentium era [and maybe explains the lack of a turbo button on my case].

Actually thats probably for the Turbo digital LED speed display.

🤣 yes I see one of my LEDs doesn't light up, I don't have a speed display as such but there is a LED with a lightning symbol next to it. Will have to look for the pins, I guess they are going to be on the back of the front panel.

That couple of cables ,which measure 5v, are only for the speed display. The turbo and power-on leds should be powered by the montherboard headers.