Was this carb fitting only used on W30 carbs or on all OAI carbs? Thanks

 

It was used on all 1970 4bbl carbs. The port at the back of the air horn was originally used to source clean air for the hot air choke. This tee fitting was created to provide a second filtered air port for the TCS valve to allow filtered air to flow into the TCS system when the valve was released. This was a last-minute design mod to avoid changes to the carb air horn. The 1971-up carbs had two separate ports.