The only adjustability in a good Aussie made stock saddle is adjusting the flocking. A good half breed saddle has NO adjustability once made. You can buy a cheaper bates stock or fender with a changeable gullet but that is it.
How do those saddles claim to be adjustable?
A good stock saddle is made on a high quality tree that will fit most horses of the same type, and you tweak the fit by either re-flocking (traditional) or padding (half breed).
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I know the Sid Hills I've seen are not adjustable.
My trooper is and I've had it widened once, but that's only possible because the pommel and cantle portion are both all metal attached to wood bars. Even then the adjustment has to be rather small (a very minor angling to give a slight increase to the bar width). I have a trooper saddler make the adjustments. Would never try it myself.