We have lots of saddles here on the place in western U.S. As mentioned, you now have instructions on measuring the size of the saddle seat. The back part is the cantle and usually the maker of the saddle will stamp the leather on the back of the cantle. Additionally, there may be a maker's stamp of the "fender" which is the bottom part that goes on the horse... the mark (if any) will be between the two parts of the fender.
Be sure to check the condition of the underside or padding, usually made of lambs wool... this is the part tht rides on the back of the horse and shouldn't be overly worn down to the letaher... todo so will soon make the horse's back sore...