There are two power winder rigs for the RB. The older model (pro) is a two piece unit. The actual motor bolts on the bottom of the camera, and the back is powered by this. OK, but adds extra weight and complexity to the operation. The backs use foam seals.
The second, and newer is built along the lines of the Pro SD. The motor and batteries are internal to the unit. There are no foam seals. I have two of these and they work great. Wish I had one for 6.45...
Both use the same darkslide.