This valve is used with Speedway's aluminum T-bucket radiator (91015600). It threads into a 1/4" NPT hole in the top radiator tank and it limits the pressure in the radiator to 15 psi.
The aluminum T-bucket radiator does not use a conventional "pressure" radiator cap, rather it uses the decorative "T" caps to seal the neck at the top of the radiator.
The arrow on the valve indicates the flow from the radiator.