You may be thinking of the rear wheel 5/8-18 lug nuts. They were assembled as unpainted items, just plated per b/p spec, purchased in bulk and set alongside the line for easy access by the worker. Think mass production line. Now, here's the poop...the fender assemblies, skin and bracket frame, including rivets and bolts were painted before mounting at the assembly line. The bolts used to mount the fenders the axle trumpet bosses were unpainted, plated as spec'd. The 9N, 2N, and early 8N models all used bolt p/n 22185-S7, 5/8-18 x 5.56" -hex head and the fenders had plain round mounting holes. Mid-8N production change saw the fenders and bolts changed. The fenders and axle trumpet bosses now had square holes to accept the square carriage bolt shoulders and the bolts changed to p/n 355310, 5/8-18 x 6.12" carriage bolt; oh, and not a 6-1/2" length as mentioned below. Think of how mass production is done. Subassemblies are moved to the line already painted and ready to mount. Fasteners made and purchased in bulk, set alongside for worker to have easy access. Again, produced in bulk, as plated, not painted pieces, assembled from a bin on the line. Wheels, center discs, and tires were already assembled. You need to determine which model year you have for fender to axle mounting bolts. The early fenders with round holes and axle trumpets will not accept the carriage bolts. The reason Ford changed to a longer bolt was that many farmers also purchased mowers and used stabilizer brackets which required the longer bolts. Now, the fastener industry no longer makes fine thread carriage bolts, only coarse thread. Additionally, lengths are standardized too so you are not going to find a 5-9/16" length or a 6-1/8" length in any bolt size -they just ain't made. You'd have to buy a 6" hex and a 6-1/4" or 6-1/2" 5/8-11 carriage respectfully and cut off what is not needed. Now, all that being said, Ford tractor repop guru Dennis Carpenter does make and supply exact-as-original p/n 355310 5/8-18 x 6.12" Carriage Bolts for the fender mount. The kit includes 4 bolts, 4 5/8 lockwashers, and 4 5/8-18 Hex Nuts, also exact-as-original with the HD thick design and 1-1/16" AF (Across Flats). $25 is well worth it in my opinion. You won't find any Industry Standard Fine Thread Carriage Bolts, in any size, anywhere else today -they just ain't made.
Tim *PloughNman* Daley(MI)