In the early days of firearms, bullets were mostly made of cast lead balls. When we look at firearms from the 1800s, they were frequently hand-made and the user was often the same person who helped build it. None of these weapons were built to any particular standard and hence, each weapon was supplied with its own bullet mold, so that the user could cast their own bullets as needed. The bullet mould seen here was primarily used to make lead musket balls, and while the date of this mould is unknown, this type is similar to those used in the late 1800s.
“To cast a ball using the bullet mould, the user initially melted a quantity of lead in a container. Then the user would close the mould and pour in some lead through the hole on top. The lead quickly cools and solidifies inside the mold. Next, the user opens the jaws by manipulating the scissor arms and extracts the bullet. This bullet is mostly spherical except for a tiny bit of projecting metal, called the sprue which is formed by lead hardening in the hole through which the lead was poured through. There is usually a thin parting line formed around the ball as well, along the line where the mold opens. The lead bullet is taken out and then the sprue and parting lines are filed off, to leave behind a spherical bullet.” (firearmhistory.blogspot.com)