Just realised that I didn't entirely answer your question.
These saddles sell new for around $250, when yours was new around $150.
Mounted means that the girth, stirrup leathers and stirrup irons would be included.
This saddle has a cut back head (the area at the pommel) so you measure it from the small silver stud on the side to the centre back of the cantle. If the saddle was straight cut you would measure it from the centre of the front of the pommel to the centre of the back of the cantle.
Cut back head
Straight cut head