I got it backwards, oops, but they are somewhat related. An american company purchased Racing Dynamics (Italian Co.) and formed Racing Dynamics Inc. The orginal Racing Dynamics (Italian Co.) is now RD Sport, where they still develop new products.
Racing Dynamics was and is an Italian company. They spawned RD Sport for the USA/Canada market. Electrodyne Inc started a new company called Racing Dynamics in the US. They since registered the trademark "Racing Dynamics" in the US and Canada which has pissed off the original company.
So if you want authentic Racing Dynamic stuff get it from RD Sport..