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Dear JM,
I'm afraid that there is no built-in option automatically filtering the choices of the filter pane.
If you want to only show the county name before the comma, you can add a calculated column County Calculated as follows to only show the county name like the following.
Calculate data in lists or libraries - Microsoft Support
Formula: =IF(ISNUMBER(SEARCH(",",County)),LEFT(County,FIND(",",County)-1),County)
Thank you for your time and effort.
Sincerely
Cliff | Microsoft Community Moderator