Commonly called mechanical pencils, these make sketching or writing in the field clean and easy. Leads can be inserted into either end, just slip the ring down to tighten around the led. These are handmade out of brass.
I give quite a few American Revolution presentations, in costume, and the Porte Crayon (small) is a perfect compliment to my leather journal with its hand stitched pages. Always lots of questions about my "gear" and this one attracts attention. If you want good quality "gear" search their catalog. No disappointments here.