Sorry to have to come back to this after a pause, dear modelers. After making the mistake of entertaining friends without masking, and recovering from what that led to - thank goodness for vaccines, boosters, and anti-virals - I got out one each of the AM P-51A and C, Tamiya B and D, Hasgawa D, Revell B/C, and Meng, Airfix, and Eduard P-51Ds. The tails of all these save the AMs are within a hair of being the same height held side by side where possible (the Meng is most strangely pieced) or measured along the rudder hinge line.
The AM kits are the same height as each other and both are a good 1 mm taller than all the others, and as the fin and rudder angles narrow with the height, the AM rudders look a bit more pointed. To my eyes, poor as they have become with age, perhaps because of my training in plastic injection quality control or acquaintance with a modeler once famous (notorious) for nit-picking shape differences, this is a noticeable difference when the kits are placed side by side.
So my question remains, is there a good reason for this 1+ mm, such as the Allison Mustangs requiring a bit more yaw stability, did AM make a mistake, did the rudder height change during production, or did all the others copy a different mistake?
If it is an error on AM's part, it can be partially fixed by trimming the 1 mm off and restoring the curves of the front and rear edges. This leaves the rudder mass balance sitting a bit high, but that to me is a less obvious problem, or the mass balance can be filled and rescribed.
Very nice work indeed, George!