You can use the Pentel guts from any of the big box office stores. I understand as I can't use those sissy 2mm and all those micro sizes. I can bend the lead sleeve in two letters not to mentioning breaking the leads half way through a letter. http://penmakersguild.com/articles/pentelpencil.pdf Here is a link to Don Wards great article PDF on using the Pentel mech. I believe his user name on this forum is It's Virgil. It is on the Pen Makers Guild article page. Pen Makers Guild Articles Page here is the list of what they offer it is a graduate course in pen making!
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I just got some bullet/cartrige pen and pencil sets from PSI. Click pen and pencil and the pencil takes a 2mm lead. It comes with a sharpener and eraser in the push button. They look nice with deer antler but haven't tried to use them myself. They were a special order from one of my customers. The lead seems quite substantial being 2mm.