What is the box cirled red on the alternator??

James is correct its designed to stop interference with other electrical components like the car radio ect.

If you have replaced brushes and the regulator and the unit still isn't charging it could be many things like blown diodes on the rectifier internally or even a bad winding on the stator or a worn out slip ring. The suppressor you circled won't cause a no charging fault.

Clean up the contact circled in blue the regulator sits on with some emery or fine sand paper. Also try to use original bosh regulators or parts. I know knock off's are cheaper but trust me you get what you pay for.

I bet it’s a wiring harness problem

Cars from 1980-1987 are all affected

Couple of things to check:

• Is the earth wire on it? Looks like it was as there was a nut loose in the pic (top left).
• When running is there 12-14v at the large red wire on the back of the alternator? Use the battery as an earth, then the block as an earth, then the alternator housing. If 12-14 on some and not others then chase earth straps.
• Is there 12v at the small wire on the alternator when running?

While you are in there check the rubber bushes for the lower mount and top adjuster, the car looks in good shape so you have likely already done this.

If you get a replacement I suggest going for an 80 or 100 Amp unit, that would most likely have been a 55A.

Hi SingV240, Of topic but what wheels are you running, look like performance miniltes? what size and offset? they look great!

Stoonly I have managed to solve part of that mystery thanks to his post here: https://ozvolvo.org/d/30111-wanted-coiloverslowering-springs

17 inches diameter, 8 inches wide "Super"lites.

Whip the alternator down to an auto electrician, they will tell you in minutes (or less) whats going on.
Good chance they won’t charge you either.