I only found 1 like that.  I found a "thumb screw" head bolt the right thread size and ground it to match.  The target bolt was basically a 3/4" round head with two opposite side flats.  I ground it to fit the slot in the plowshare and the bracket it mounts to.  Most of the other "special" bolts look like standard hex head bolts.  The only thing special about the ones I checked were that they were fine thread.  But that doesn't seem like a reason to use the term "special".

They were some type of a shear bolt. 

Two bolts below were 3/4 hex head bolts both fine threaded and labeled "special".  Would those be "shear" bolts?  Or would they be higher strength bolts?  The second one is the upper bolt of the U bracket.  I used grade 8.  I don't think it matters much at this point, was just curious.
 

CA Pickup Plow
 

#52? Plow

321893 then the numerical book says use 916290 which are grade 5. 3/4” x 3-1/2”

321896 numerical book says to use 916292 which are grade 5. 3/4” x 4-1/4”

Hmm, grade 5 is roughly equivalent to 4140 which most everything else was, I thought.

Interesting comparison:

https://www.makeitfrom.com/compare/Annealed-4140-Cr-Mo-Steel/ASTM-A387-Grade-5-S50200-5Cr-0.5Mo-Steel" rel="nofollow - Link

The two bolts referenced were normal hex bolts (I'm reasonably sure the bolts I removed were original), only they were fine thread.  

All "special" really tells you is that technically there is at minimum one small thing that doesn't line with the standard. Sometimes you can get away with an off-the-shelf bolt built per whatever standard, and sometimes you can't, but the manufacturer (AC in this case) typically had some reason for choosing that special bolt in the first place. It could just be something that made it easier to assemble or it could be something that kept it from failing in the field.