It also fits f103 chassis. Great upgrade for your chassis performance. it gives more torsional resistance. Standart f103 mount hasn't got enough room for brushless motor, but it has. You can change motor position easily for different pinion sizes. You can also change ride height with adapters.
This Blue Anodized metal motor mount is a must for any high powered F104 enthusiast.
Why it isn't included in the F104 Pro Kit, is a question worth asking Tamiya, but if you run high powered motors or just like your motors to live a little longer, then this is 'a must'. The added Heat-sinking will keep everything a bit cooler as well as look Fantastic.