This is one blue Schmidt P8126 Capless System roller ball pen refill. This is the short model version, and the stainless steel tip has a 0.6 mm ceramic ball. It has a one year cap-off time, meaning that the ink does not dry out even if left uncapped for one year. This refill is compatible with the Retro 51 Tornado pens listed under Compatible Products.
Note: This is the Schmidt P8126 refill, which is 3.9" (10 cm) long. It is not the same as the Schmidt 8126 refill, which is 4.3" (11 cm) long.