The valve you show is the type used originally. It is cable operated and uses the copper coil to sense the temperature in the heater core. This valve is very hard to find. The one being offered by the vendors is a vacuum operated valve with no temp sensor, and needs an additional vacuum switch to make it cable operated in addition to a vacuum source. I'm not sure if these were offered as replacements or are for another application. The same part number is used on both (in error?).
To test your unit as shown, block one of the main tubes and introduce 15 psi air pressure into the other while the unit is submerged in water The rising bubbles will pinpoint the location of your leak. If it is at the valve, there are seal kits to replace the seal (not an easy job), if it is at the point where the valve attaches to the core, then the oval rubber seal is bad and can easily be replaced. If the core itself is leaking, it can be replaced with a reproduction or repaired by a good radiator shop.