Case 580K component/cap identification

Hi, I just bought a 580K, and Im trying to ID what this cap is called so I can buy a new one, Ive run a 310J for a while but this is my first Case, waiting on a user manual or else I'd have already looked it up. The cap in the red circle is broken, and as far as I could tell it looks like the cap for the coolant tank, but when I go and type in case 580k coolant cap I get results for the cap at the top of the radiator that I circled in blue. Any help would be great, the reserve that the cap...

Case 580K component/cap identification

Case 580K component/cap identification