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.
It's Mk14 lead computing gyro gunsight. Designed by the Brits, and credited with increased accuracy / higher kill ratio (it took a lot of the guessing out of shooting). The padded part with "No hand hold" stenciled on it, was in fact padded to protect the pilots face / head from impact during a crash or crash landing. The Mustang did not have modern inertia reals on the shoulder straps (they could be locked up manually by the pilot).