how to determine spring strength without stamp?

A quick learner from Brentwood CA says:
I have Milgard single hung windows and I think the series 385 channel balance (S385-33) is the right one but I'm not sure of the weight since it is at a rental home so I can't weigh it and there is nothing stamped on the balance that I took out of the window.

The size of the window sash is 36" x 36" so I'm not sure what weight option I need to order. Can you tell me what the weight is or which option I need to order?
User submitted a photo of a window balance.
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Tom from SWISCO responded:
I'm afraid we cannot determine what the weight of the balance is based solely on i ts size. Can you double check to make sure there is nothing printed on the metal channel of the balance? Aside from weighing the window sash, that's the best way to determine what option you have. Otherwise you will have to weigh the window sash.
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