Question

What method of manufacturing, introduced in the early 1800s, made mass production possible?

Answer 1

The method of manufacturing that made mass production in the USA possible was the implementation of interchangeable assembly parts leading up to the moving assembly line for large scale production.

Explanation:

These innovations originally took place in France, where Honore Blanc proposed the manufacture of interchangeable gun parts, but he was shunned by the industry because the manufacturers of the time relied on being unique.

The Americans liked the idea so they could become independent in artillery manufacture and so promoted interchangeable parts in their own industries.

Although assembly line techniques had been developed as early
as the `12th` century in Venice, the implementation of interchangeable parts allowed American industry to expand on the idea and build huge high-output factories.

The first assembly line for large scale production was the creation of the Olds Motor Company. The success of this venture inspired Henry Ford to develop a moving assembly line to maximize production of his own brand, and large scale production took off from there.

There are images and details here:
http://robohub.org/the-evolution-of-assembly-lines-a-brief-history/