First, let me say that there are changeable gullets, and there are changeable gullets. (There are also changeable heads - note the Albion Genesis models - but that's a different kettle of fish, and not one that I know much about yet.) So ... there are quite a few saddles that offer the changeable gullet system - Anky, Pessoa, Collegiate, Wintec, Thorowgood - but the ones I'm most familiar with are the Wintecs and Thorowgoods, so I'll confine myself mostly to those two, though I will say that I think the Ankys and Pessoas have one of the easier systems to work.
First, let's take a look at the basic gullets. Here's a Wintec standard extra-wide plate:
And here's one of the Wintec Wide plates (it has three: wide, wider and widest; this is the wider):
Now, here's the Thorowgood xw:
Thorowgood offers two different types of plates: The S bar (bottom, for square cantled saddles) and the R bar (top, for round cantled saddles). The R bar, which has longer points, is better suited for a horse with a good wither and is used in their standard and high-wither models (and the standard Showjumper saddle); the S bar, with shorter points, is better for a lower-to-no wither, broader horse.
Now, here's the Wintec medium plate sitting on top of the Thorowgood medium plate.
And here's the Wintec medium plate compared to the head of a Black Country tree:
Have you noticed anything about the Wintec plates? There's something unique about them, something not found in the head plate of fixed tree saddles or on the gullet plate of any of the changeable gullet saddles. Here are some hints, in case you haven't found it yet.
Here's the Thorowgood medium gullet plate:
Here's the Black Country tree:
And here's the Wintec medium plate:
And the Wintec Wide "wider" plate:
Before I divulge the unique feature (which you've probably already recognized), let me say that the issue that I'd run into rather frequently when fitting the Wintecs was that even when the angle of the tree point (the lower part of the plate) agreed with the horse's back, the fit just wasn't right. At first, I'd put it down to the CAIR panels, which need to have the rider up to really evaluate fit ... but getting the rider up wouldn't improve the issue. Switching out the plates didn't make the fit better, nor did flocking or shimming. For the life of me, I could not figure out what the problem was ... until I received an e-mail from a fellow fitter that mentioned "that damned kink" in the Wintec plates.
I'd handled those plates for years, but never really looked at them. So I grabbed a plate, put it on my bench, and spent a few moments studying it; Yes, there was definitely an inward kink about halfway down the leg of the Wintec plate ... and the lower part of the plate flared out ... This creates two different angles in the gullet plate - one above the kink, and one below it:
And when you compare these angles, they're quite different:
Now, when the gullet is installed in the saddle, the kink corresponds roughly with the top edge of the panels, so technically the lower part - which would be the tree point on a fixed-tree saddle - is one angle, and that angle should be parallel to your horse's back. However, in my experience, this design creates the potential for a great deal of pressure right at the top of the panel. On some horses, this doesn't seem to create issues, but on some - especially those for whom the panel placement isn't quite perfect - it does create a substantial spot of pressure ... and there's nothing that can be done to correct it. And when I see the "straight leg" on other changeable gullet plates, and on the head plates of the fixed-tree saddles, I look at that inward kink and wonder "Why?"