Does Anybody remember Combat Flight Simulator WW2 Europe Series ?
In the P-51, there is a "something" with "No hand hold" text on top of it
What is that ?
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.