Spiral Sash Support Balances for Non-tilt Windows

I am trying to replace the spiral sash support balances for some old windows and I am unsure which series and brackets to select. I know they are 5/8", but the old ones don't have any identifying markings. I have included pictures of the end pieces that seem to be the main differences between the series, and would appreciate any guidance on which to select.
Thank you.

The tube of my current spiral balance measures 26 3/4" or 68cm. How do I select whether to get the shorter or longer balance? My windows weigh about 10lbs each.