Yeah you have a mystery relay box on the firewall with non original wires. If you want to try to have one of these smarter people try to help you trouble shoot it thats fine. But what I do when I get a new to me vehicle is remove all the non stock added on after market stuff to get back to bare bones as close as possible. It becomes really hard to trouble shoot and fix previous owners hack jobs.
I for one would take that fuse box down to the machine shop and have it sand blasted and then find a cover for it. Probably cost you $10 plus a top maybe another $10 if you post a ad in the wanted section. Check for cracks in the base of the fuse box if you remove it.
As far as the mystery box you should have 4 relays on the inner right fender. horn, head lights, e1 or e2, voltage regulator. The link you posted is for a L521 which I dont see the e1 or e2 relays which I think is for the dual points distributor. Here is a wiring diagram for the pl521. I know I'm not being much help here. I'm not great with wiring. When I got my red 71 521 the voltage regulator wires had been cut along with everything else so I just replaced the whole engine bay harness with a good used uncut one for $40.
Click on diagram to enlarge.