The fourth is a grade 7 marking, the other three are grade 8. From what I've found, the bolt is 380459-S(2) (Motorcraft BB390AC) which should have the following measurements - 1/2-13 X 1 7/8 in L, 13/16in thread length, knurled, 3/4 hex drive. Per Ford Car Parts, this bolt was used from 67-70 and on service shafts for earlier years. I could find no reference to grade but it may be a minimum requirement, or an engineering authorized substitution. The other head markings are typical of what supplier Ford used. I have a mix of grade 6 and grade 7 used on 64-66 so Ford used what would work. To get year (and month), plant and model specific, Jeff may have pictures.
If you question is what to use, I would find four matching.
Jim