[Mgs] DPO butchering of Alternator replacement?

[Simon Matthews]

I have a '71 MGB-GT on which I am replacing the alternator.

According to Moss Motors, the replacement only uses 2 pins of the
3-pin connector (the instructions  are badly written, but this is what
I understand from them).

Looking at the existing installation, I see an alternator that has the
new 3-pin setup. The wiring on this 3-pin setup has a thick brown wire
(presumably the Positive wire) connected to one terminal, a medium
brown wire (presumably the "A" wire) connected to another terminal and
a thin brown/yellow wire (presumably the Indicator wire: "D")
connected to the smallest terminal on the Alternator.

Also hanging from the wiring harness is the old 3-pin connector, which
has one wire still attached -- a thin brown/yellow wire.

Was there ever an alternator in which the connection of these 3 wires
would have been necessary? Is the connection of the medium brown wire
just redundant, or could it be harmful?

Any suggestions?