Those are side panels. All body coats, i.e. dress coat and morning coats are cut like that. Rarely seen nowadays except on military uniforms. Patrol jackets, mess jackets, tunics and service jackets for certain...
After lurking for a few years, this post has finally made me create an account.
As I also reside in Ontario, I can highly recommend Trend Custom Tailors in Toronto. They have experience making
detachable...