Need to replace window coil balance

A home owner from Ontario, Canada says:
hi I need to replace single hung sash window single coil balance,

window weight : 18.10 lbs
window size : 24 1/4 inch (w) x 28 1/2 inch (H)
notch size (inside) : 1 1/4 inch +

question: should i choose 1 pair of 9lbs balance or 10lbs balance

thanks
Ran
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Paul from SWISCO responded:

Hi there, Ran. Before we can answer your question, we would like to ask you a question, if we may? What would the stamped number happen to be on your old coil spring that's attached to the pivot shoe? With the answer you provide, we will be able to give you an answer in return. Thank you.

A home owner from Ontario says:
hi Paul,

thanks for following up on this case, unfortunately, there is no any stamp on the existing coil spring, as the picture shows. Is there a way to identify the coil without it?

Thanks,
Ran
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Paul from SWISCO responded:

The stamp is usually on the face of the spring itself, near the tip. It wouldn't be on the cover or shoe or anything. Your pictures above don't show an angle that would display where the stamp is.

We can go by the sash weight, but that's where things get tricky. Your old pivot shoe appears to match our 15-140, which is compatible with our Series 150 spring. However we do not have a 9 pound option for the Series 150 spring. That's why I wanted to confirm what your stamp read.

We do have a the S152-9, which can support 9 pounds (so having one on each side would support an 18 pound sash). However I don't think it will work with your original shoe, so you will also need a new 15-158 pivot shoe to work with it. You will also need a 33-072 screw to attach the new style spring to the new style shoe. Hopefully this makes sense. 

A home owner from Ontario says:
Sorry for the late reply Paul, after i took the coil out, it indeed has a number 9 printed on it. Does it change any recommendation from the last post? thanks very much!
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Paul from SWISCO responded:

Nope! It all still stands.

A home owner from Ontario says:
Thanks so much Paul!

Ran
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