Can anyone explain to me what appears to be the highest grade bolt (9?) on the entire car (my 72 anyway) is used to simply hold a dust/debris backing plate to the front disc spindle? Then there are the 2 other smaller bolts near the bottom doing the same, smaller grade, but they hold the steering arm to the spindle.

I then order a repro set of the backing plate/splash bolts for the halibut to see what they look like... and they are piddly narrower head things with no grade markings at all on them.

I don't get it.

 

I've never heard of "Grade 9", but that upper bolt also holds one end of the caliper mounting bracket.

 

Grade 9 is a step above Grade 8, as you would expect. For example, they were used to hold the ring gear on Dana 80 differentials. Where the same size would be 120 ft lb with 8's, the 9's were torqued 200-240 ft-lb.
http://www.mcmaster.com/#cap-screws/=a035k1