Could this spacer have been needed solely for brake lining wear? This was added and nearly all the adjustment was taken up on both ends.

I can’t really see any lining through the inspection port. I’ll have to get a stand or something to pull the wheel off, I guess.

Possibly the wrong cable "made" to fit?

I think that Yamaha changed the design of the front wheel hub and brake mid seventies, but not sure exactly when. You might have a new style cable on old style hub, or vice versa.

No, doubt it. Better pull the wheel & have a look. If the linings are good then reset the brake arm on whatever spline you need to get rid off all that cable play. If new shoes needed then same story, just set it on whatever spline is best to get rid of all the play except for a bit that the adjuster can cope with.