Product review (5 stars): Perfect replacement
This also works with my Aprilaire 600 bypass humidifier.  It works great and handles two wire, low voltage connections.  I installed on my wall in under 10 minutes.  No problems.  It looks better than the old brown one I had on previously, since 1980.  Not to mention Aprilaire's customer service is amazingly fast!  They email answers to me within minutes during business hours.