This antique mechanical pencil made from bone most likely by Sampson Mordan & Co in the 1820s or 30s.
Sampson Mordan and John Isaac Hawkins patented the self propelling pencil in 1822 and marketed it as the ever-pointed pencil. The style of the metalwork is identical to known pencils by Mordan and the letter M is stamped on the barrel. In a system used by Mordan from at least 1828 the M means the thickness of the lead is 1.05mm which they called Medium.
There is a screw top on the end of the pencil for storing spare lead inside the pencil. I have fitted a short lead so the pencil is ready to use.