This 1928 Security #5 is Twist-fill in Red Mottled Hard Rubber. Inside the light yellow Bakelite cap-top is the check protector with it’s rotating blades used to perforate the check so it could not be altered. GFT, including the spring loaded clip. Medium, extra-flexible nib. New-old-stock. Mint. Shown with cap on top of cap removed so you can see the check protector mechanism.