The other tree was their "D":
The difference between the D and this one can be seen below:
The front and back have the same curve (at least to my Mk1 eyeball), but it is flatter between and I'd guess a tiny bit steeper in angle.
I was concerned that there wasn't enough flare in the front, but the D barely touches her in front...contact ends about halfway between the "D" and the leading edge. Should it?
FWIW, the D is a near perfect match for the bottom of the Circle Y Mojave, which is an "Arabian" saddle. Mia & particularly Trooper (whose back is close to Mia's in shape) have logged a number of miles in the Mojave without sign of soring or discomfort.
This is a very frustrating part of western saddle buying. Mia actually moves very well in the CA/Martin, which is a flatter angle than the D, has more flair, and has portions of the tree not even in contact with her - her sides are dropping away from the tree. Yet she moves well in it, seems happy...but I've had the saddle slip once, and I could see it happening again.
I'd love a saddle that fit her well, but FINDING one is really tough!