Some of the older cars had right hand thread on one side of the car and left hand thread on the other. The "R" could designate right handed threads but not sure what car or age.
if I remember right the older Chryslers used right hand on one side left on the other
take a look here for the answer: https://www.google.com/search?q=vin...KHT3YCZcQ9QEwAHoECAIQBQ#imgrc=RQeoybAYHZJBRM:
My Plymouth Barracuda was the same way.