[Healeys] Fuse block terminal A3

[jmsdarch at sbcglobal.net (john spaur)]

1962 MKII BT7

My wiring diagram shows three white wires attached to fuse block 
terminal A3. My wiring harness has four white wires.

One goes to the overdrive relay, one goes to the S.W. connector on 
the ignition coil and one goes to the overdrive switch.

The forth wire is in the wiring harness and follows the same route as 
the wire that goes to the overdrive switch.

My original harness only had three white wires.

What is the purpose of the forth wire?

Thank you,
John Spaur