This is what the vent looks like if it is a Forespar fitting:
BTW, the joker valve fits in a 1/4" compression fitting like it was made for it. I've made my own vac breakers for fuel polishing system header tanks and bilge lines where I didn't want to use the whole Forespar return bend. You can buy just the cap and valve from West Marine or Forespar.
I have also used the Forespar vented loop on a head line for neat appearance but didn't need, or want, the vac breaker function so I just replaced the valve with a small disk of thick rubber gasket material.