The A70 back is easier to modify than the RB67 back, and the process is reversible. However, you will need to purchase a custom-made wheel such as this one:

To replace the original sprocket with your new rubber-lined wheel, simply prevent the sprocket from turning and loosen the silver ratchet wheel just above it, as in this old illustration:

The ratchet wheel loosens counter-clockwise, just like a standard screw. Once you’ve removed it the sprocket slips right out and you can replace it with the rubber wheel. Replace the ratchet wheel and you’re done!