This pen refill upgrade kit is designed to fit the Mil-Tac TDP1 and TDP2 Tactical Defense Pens. It features a black ball point pen refill that can write upside down, on wet paper, and in all weather conditions. The ink is waterproof and will work in temperatures ranging for -30F to 250F. The Mil-Tac TDP Refill Upgrade Kit is designed to work with Rite in the Rain outdoor writing supplies in any environmental conditions.