This article is from the Winter 1977 issue of Electronics Today International.  The article contains an interview with John Stears who talks about the workings of the R2-D2 and the Dome, Box, Umbrella, and Stick droids.

"Two versions of R2-D2 were made, one for Kenny Baker to fit inside and the three-legged radio controlled version."

"R2-D2 has three forward speeds, but no reverse, and is steerable."

"Steering is via the front drop leg, with proportional self centering servo unit. The twin wheels in the steering foot remain parallel to the other wheels during turns."