Actually, George, I was just recently wondering about that bokken example of yours. In fact, I do know by feel exactly how long my bokken is, and in a kata, I can intentionally miss someone by an inch or two. I have no idea why I can do that. I bought an Iwama style bokken last month. I picked it up and I knew it was three inches longer than my other bokken. And in fact, it is. But I knew it without a tape measure. How did that happen? What is body mapping with a ken?
It's really amazing when you realize what your brain is really doing... I can pick up a weapon, just as you did, and know just from feel where the tip is. While a beginner might be able to do that plus or minus an inch or two, I know within a half an inch. Practice effects ones ability to do this mapping more precisely. I don't accidentally smack my partner just because I picked up a new weapon.
That's an astounding thing! The brain virtually instantly incorporates the bokken into your body map the moment you pick it up. This happens when you grab someone as well. They become part of your body mapping system and you, theirs. Physical tension seems to interfere with this which explains party why muscling people doesn't work so well. Anyway, there's a lot of interest in this book which has implications for what we do and how we train to do it.