Today it would be unusual to think about making each bolt by hand to assemble a project. Yet that is what was commonly done by rural Blacksmiths prior to 1850. If a blacksmith had the right tools he could make a huge assortment of nuts and bolts.
The most common style of bolt was a square head. That could be held easily with a wrench and was easy to forge with just the hammer and anvil. But when building wagons and sleighs he needed a lot of other kinks of fasteners. Carriage Bolts have a rounded head to prevent them from catching on things and have a square spot on the shaft to hold in place in a square hole. That prevents the bolt from turning and allows a nut to be threaded on the other end. Plow bolts have a head that is both square and tapered so that it sits completely countersunk in the plow mouldboard. If the head were raised it would prevent the plow from working smoothly and be damaged by the dirt.
What kind of tools can hold a bar of steel while the head is being made? Bolt headers! The simplest form is just a flat bar with a hole in it. The best of the old Blacksmith made headers are pleasing to use, to hold, and to look at. I have a number of headers from a rural Adirondack farm shop that made and repaired wagons and sleighs. Most of the dozen header set are "Dog Bone" headers, with a different die on each end.
This header has a face for making 1/2" carriage bolts on one end, and for 5/8" carriage bolts on the other. It was made by forge welding the bar back onto itself to made the ends, and then forged thinner and square in the middle.
You can see in the picture above of the underside of another header in the set. Note the evidence of the folded ends and the forge welds.
Forge welded headers aren't just a product of the distant past. I was lucky to find a header made by Donald Streeter. It has a forge weld the whole length of the handle, as well as a steel face forge welded on working end. This was probably one of his working tools. He made a lot of large hinges, and this would be the right size to make 3/8 carriage bolts to mount hinges on a building.
He was one of the premier American blacksmiths in the 1970s doing colonial restoration hardware. His shop in New Jersey was featured in his book on blacksmithing, Professional Smithing. That book is being reprinted again by Blue Moon Press, and is a good reference for any Blacksmith reproducing traditional Colonial ironwork.