In this circuit, there is a wire between M5-1 and M5-3 (at the bottom). It's a little plastic connector with a wire in it that you can pull out.
I've been trying to debug a power issue after recapping this 1980's power supply. At first, I forgot to put that connector back. Some of the reading looked correct, but others were too low. When I added it back, the formerly-correct readings jumped to ~1.5x.
What purpose does this connector have in this circuit? Why would you want it connected? Why disconnected? Why isn't it just permanently there?