Thomas Herbert Martin's design of 1886 envisaged a single "...peculiar shaped piece, which, upon being bent into the desired form gives the base, sides, cloth plate and presser arm of the machine..."
Even the earliest known models have a separately factored base, they do, however, feature an integral stitchplate.
The patent specified that sheet brass, iron, tin or other metal could be used for its construction. When it came to production, brass was the material chosen.