Victorian Lady Waterman's
1920's Ring top Gold plated , beautifully engraved Propel 1.1mm lead
The earliest pencils, such as the Mordans, are simple, propelling pencils. The lead is manually pushed down a tube of matching diameter and is friction fit. A small rod inside pushes the lead forward, as needed, usually with a twist action mechanism. When done writing, the user twists the mechanism in reverse, and manually pushes the lead back down into its socket.
Some of these pencils are simple, some are fancy. With lead-storage compartments, erasers hidden inside the finial, or even finial-mounted engraved jewels, there was something for every income level. But until the early 20th century, they were generally all just plain, propelling pencils.
Common Vintage Lead Sizes
The most common sizes of lead for vintage mechanical pencils were 1.1mm and 0.9mm. 1.1mm leads were frequently referred to as "thick" lead and the 0.9mm size was often referred to as "thin" lead. Of course, today, there are much thinner diameter leads made, but we're talking antique pencil leads!
For your reference:
1.1mm = 0.046 inches
0.9mm = 0.036 inches