Stressmerchant
Was the bobweight there for stability reasons, or was it the flutter balance weight?
It appears that the P-51 had a bit of a tendency to experience stick force lightening at higher g levels without the bobweight. The Technical Order specifically states,.
PURPOSE.
To prevent reversal of the elevator control stick forces during maneuvers
So the bobweight was there for stability reasons and not for phugoid reasons.
Any tendency to stabilize the elevator was an added bonus.
Ideally you want the counterbalance mass evenly distributed and firmly attached to the control surface, but in practice this can be difficult. As I recall, my F-4 had rudder mass balances in a shielded horn near the tip and out of sight in the fuselage.
If you have time, read into this link about Strega and what happened when they switched to a "tube" engine.
Tiger Paws