Square screen spline for a Pella door

A quick learner from NJ says:
I have a Pella sliding patio door screen that is 40 years old. The spline is grey and square in shape and the channel it fits into is square. Pella said Swisco is the company that they used for the spline but I don't see any square spline on your site.
Pella also no longer has the specifications on it. They directed me to item 59-150S (which I bought a year ago) but it is round and Pella doesn't think it will fit. Last year, this door was re-screened and the Pella technician used the original spline from 40 years ago. A house sitter let our dog go through the screen and it has to be re-done again! This time I am worried that the 40 year old spline won't make it a second time. Can you help me? Please, I'm desperate!! Thanks
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Dave Sr. from SWISCO responded:

I know exactly what you're referring to. The bad news is that the Pella screen spline you need is no longer available. It hasn't been made in a long time. 

The only reliable option is to reuse the original square spline that's in the door now. A 1/4" thick spline like the 59-250 might work, but there's no guarantee. It would be best if the spline was serrated.

Otherwise reusing the old one is the only tried and true option available to you. They just don't make anything like this anymore.

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