Sash Weight Range
A quick learner from Ottawa, Canada says:
September 13, 2016
the balance in my window, shows a stamp that says 18A
Tom from SWISCO responded:
September 16, 2016
Thanks for posting. I see you were looking at our S351-19. Is this what you have? If so, I recommend the 1810 option. A stamp of "18A" usually corresponds to this. That said, you can always weigh your sash to confirm.