At ROBODEX 2003, Honda showed off several color variations of ASIMO including white, red, blue, green, and silver. However, the silver ASIMO is more than just an eye-catching paint job.
https://archive.org/details/movi017
On the side of the head and the back of the backpack, there is a letter “R”. The latest version of ASIMO at the time of ROBODEX 2003 had a top speed of 1.6 km/h. Honda gave a demonstration of this walking speed using the green colored ASIMO. Next, they showed ASIMO R walking at 3 km/h.
https://archive.org/details/movi018
The feet almost looked glued to the floor. The ankles and legs are noticably different (AKA thicker) than the current ASIMO. Also the backpack seems taller and rounder.
ASIMO R was only shown on the main stage and was not at the Honda booth. This was the only time this variation of ASIMO was ever shown.
On December 15th, 2004, Honda unveiled a major revision to Asimo.
https://www.youtube.com/watch?v=fGGeKIklnHs
Asimo(2004) can walk at up to 2.5 km/h, and can run at 3 km/h. The ankles and lower legs of Asimo(2004) look very similar to Asimo R. The walking (and running) look more natural as Asimo(2004) uses it’s torso and arms to counteract the inertia caused by the legs.
More info on Asimo from 2004 to 2007 can be found here