Can you post the Amazon link to the cartridge you received? You should be able to find in the 'last 30 days' orders section of your Amazon account.

Here's the 2013 Unicel cartridge replacement guide: http://www.unicelfilters.com/pdf/replacement_guide.pdf

It does list the C-8409 cartridge as the correct one, but it also shows that cartridge as a 100sft cartridge, so I'm not sure what is going on.

Here's what I'd do:
1. Compare your current cartridge with the new one. Dimensions and end forms should be identical.
2. Count the pleats on your old and new cartridges. If they are the same number, go for it. If your old cartridge has 10% more pleats than the new one, something is going on. Call Unicel and ask => 818-678-0400

Please let me know what you find, or if you have any problems getting answer from Unicel. I can call them if they won't help you directly.