> Think it would be unusual for the lower jaw to grow
It is not that unusual. Several have already commented their dogs have changed long after puppy age. Skeletal growth into maturity is not limited only to the major bones of the limbs and body, the mandible like any other bone in the body can and will grow at different rates, and may not stop till a dog is 2 yrs old and beyond depending on the breed. As I said in a previous post true undershot is different to lower incisors popping forward.