"I had no idea about the fuse 15 other use - sounds like you have some experience to learn of that?"

Sorry -- that was a total brain fart. No other function known by me. The problem was that I was looking at the '87-'88 greenbook you donated some time ago. That edition has only the rather cryptic Fuse 15 reference which seems to imply the some other fuses (16 maybe?) have multiple functions. Now I see from your posted pages that they did add the F16 info to the '91 edition.

I wonder if the '91 has another error found in the '87-'88, where the Main light relay, as shown in the "High/Low Beams" diagram, not only has the incorrect 87a labeling but has a line connecting 87 to 87a?

I still see these greenbook diagrams, while helpful, as being the product of small bore person power (maybe a step up from entry level?) and thus very suspect.

As for that unique Main HL relay, the Quick Reference Parts Manual shows that PN1324749-9 was also used as the:
• '76-'77 Fuel pump relay (why the need for 87, 87b outputs isn't clear to me)
• '77-'81 Lambda relay, as Phil mentions, powering the Freq. Valve from 87b

Also, the Bosch numbers added below the relay (124) in my sketchy diagram have the "correct" '87-'87b configuration, and an 0-332-015-013 is working well as my 940 Main HL relay. So there are plenty of Junk Yard options for this puppy.
The real deal 1324749-9 is $19.69 at Tasca Volvo. The Bosch 0-332-015-006 is $16.96 at FCP.

P.S. Your latest pics above should be most helpful the next time this question comes up.
And it surely will.

--
Bruce Young, '93 940-NA (current), 240s (one V8), 140s, 122s, since '63.