Stonemasonry pencils were mentioned at this site before I had ever seen one.
Here we see a 240mm version from Bleispitz (the 0341), and a 300mm version from Lyra (the 331 Profi).
With extremely hard 6H leads, they are meant for writing on stone and rock surfaces. Mr. Slate probably used one.
My thanks to Gunther from Lexikaliker for assistance in acquiring these pencils.
This is the third of a four-part mini-series. Tomorrow: Lyra Profi copying carpenter pencils.