What is this "No Hand Hold" thing in the P-51 Mustang cockpit?
It is part of the gyroscopic gun sight:


Source: WarRelics Forum

It is not a "hand hold" because you don't want to damage the sight by grabbing the scope to situate yourself in the cockpit (getting in/out) or otherwise pulling on it. Since it is aligned to your guns, you would want to avoid misaligning it.