Rr stands for register ratio. It is not the ratio of the disc to the least significant digit. It is the ratio of the first gearwheel that engages the disc worm to the least significant digit.
The ratio of the disc to the first gearwheel is the Rs, or shaft gear reduction, which I've never seen on a meter nameplate.
Source: Page 3, Pocket Guide to Watthour Meters, 3rd edition, Alexander Publications
All of this is irrelevant except to understanding the guts of your meter. You need to know the Kh in order to test the meter and to estimate demand. You only need to know the Rr if you plan to replace a register.