A triatic stay is what links the tops of the main and mizzen masts. If you look up Vindo 50 images on Google, you will see that the photos of the ketches’ rigs do seem to have triatic stays. They can be used for setting a sail called a “Mule” if the wind is right and you plan on holding course for a long time. (Mules are a hassle to tack or gybe; they need to be taken down and re-set.) People sometimes also use their triatic stays for SSB radio antennae.