>hammer heads by the pound
The early ironmakers would have been happy just selling chunks of iron, that's what they were making, and that's all they wanted to sell, but , a chunk of iron isn't all that usefull to the average person, and there a limited number of folks around who could make other things out of it.
So the iron makers bacame foundries as well, making pots, axes, hammers and other simple shapes, just as a way to be able to sell their iron locally. Since a pound of iron is the same no matter if it is a chunk, ingot or a hammer, it seems to have not mattered to them much, and became somewhat traditional to sell many of those things by weight...
Now if only I could buy Snap-On tools by the pound...