Response to Hi Kate - The 4600-1474 (191474) would be used on FNS Plus filters manufactured after 05/21/05 and the 4656-06 (51005000) would be used on FNS Plus filters manufactured before 05/21/05. So if you know roughly when your filter was installed then you could determine which o-ring would be needed. We have since updated our page to reflect this information, thank you for bringing this to our attention.

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