Hobby Company the tamiya importer have never been interested in importing the TRF range of cars and spares, as to the reason only they can answer that one!
There are many differences between the 503 and 511, best to have a look at some pictures of both chassis on line as it's very clear (Ali chassis, floating servo mount etc etc)
If you have the budget the 503 is a superb bit of kit and bang up to date for lipo etc
Moggers on this forum has a 503 for sale, drop him a PM and he can send you details of his car, I've seen pics and it's nice!
