Some additional information ...
I use two of these
, one in the headlamp bucket, one under the seat (because most electrical components are in either of those two areas); I connect individual component 'ground' wires to the nearest "4-way".
I connect the two 4-ways with two lengths of 28-strand, those connected either in a ring terminal on a cylinder head stud or in one of these
above the engine with a third wire to a cylinder head stud.
I connect the 4-way under the seat to battery -ve (on a negative-ground bike) also with 28-strand and through a 15A blade fuse.
If you plan to use the standard ignition switch, be aware it isn't the one illustrated
in the '72 T150 parts book,
that's the simple on/off ignition switch not
fitted '71/'72 except to the T100 ...
However, if you enter the listed "39565" number with "lucas" into your preferred internet search engine, while it'll return various different 5-figure number, the switches should all have four discrete terminals arranged in a circle:-
... this is correct and #1 to #3 match the terminal numbers in a '72 T150 wiring diagram.
Btw, the bikes that used those switches connected the headlamp on/off switch to terminal #4, your bike has that switch connected to the Brown/Green wire to the pilot bulb. If you don't connect anything to #4, be aware it's 'hot' when the key/switch is turned fully-clockwise.
Ignition switch waterproofing
If you use either Lucas ignition switch type:-
. '81-on twins waterproofed the key end the best, using Nut 97-7120 to take Cover 60-7335 that clips over the lock end. You can also fit Cap 97-7123 to the key to cover the lock end when the key is in the lock.
. T160 and '79/'80 twins aren't quite as good but at least improved on the original '72 chromed ring "Nut" mounting the switch, with Nut 97-4589 to take Cover 60-4
. 2-position switch has a moulded rubber 'boot' as standard to cover the terminals.
. 4-position switch, T160 and '79-on twins mount it inside a waterproof box with the idiot lamps. However, Norton didn't, instead covering the terminals end of the switch with 06-4891 or 06-5723; risking stating the obvious, either'll require 'flag' female spade terminals on the wires.
. Nevertheless, I drill a small hole in the terminals end of the switch, at what'll be the lowest point when the switch is mounted; any water that does get into the switch can drain straight out, rather than swilling around inside for ages corroding the bits inside.