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You can use AU 5.6.1 on Lion, I have used it from Snow Leopard to Mavericks.

Try this : move AU 6.3.1 from the /Applications/Utilities folder to another place.

Try launching AU 5.6.1.
I can't see any reason it would not work.

I feel you opened 2 threads simultaneously, not a good idea.
 
AirPort Utility 5.6 should only be used to configure older base stations that are not supported by AirPort Utility 6.x or to configure settings not available in the newer version like transmit power.
 
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