I can imagine exactly what that thumb would feel like if it was integrated with my nervous system. Wondering how common this is.

When I was a very small child, I was fascinated by the sensation of different body parts - the way touching each finger in turn compares to the next, and the way the opposite side has a similar range of sensations. It feels like I know exactly how it would feel to have a sixth, seventh finger in sequence with the others. I imagine that having a second right arm grafted below my current one would have the 'right' but 'lower' feeling and I could use it immediately. But there's no way to test whether this is just a childish imagining that has stayed with me.