Thank you for posting. It's hard to say whether you have a Series 500, or a Series 550. It depends on the length of the spiral.
The best way to determine the length of your spiral is to place it on a flat surface and push it all the way down, then measuring the exposed spiral. If you let us know what this measurement is, we can help direct you to the appropriate Series.
The spiral is 29 1/2" when fully pulled out, 1 1/2" when pushed in. The overall length when pushed in is 31".
Length of the metal tube portion only is 29"
I am not able to get the sash out of the frame but best I can do to weight the sash is 14 lbs.