Hey Zedn, I am not sure if this is what your looking at. Its a CSR in line filler neck, I drilled and tapped for the steam line to come in under the cap and then there is an overflow (above the cap seal) that runs to the tank. It connects at the top of the tank which has an internal pipe to the bottom to work as a recovery.
When filling up you can see the coolant running out through the steam line (and with the volume that was coming out I am sure there is no air trapped in there), once all full I have topped up once and it works no worries.
Inline Radiator Hose Filler - 1.5 to 1.5 Inches - Cooling Products - CSR
