A couple of observations.

There is a Benjamin Davidson on the same census page with your William S. and Mary A. Davidson in 1850 in Hamer Twp. He is married to Delilah (who is Delilah Jones in the marriage list I posted) and her 74 year old mother(?), Anna Jones, is living with them. Both William S. and Benjamin were paying taxes in Salem Twp. up until 1848/49 and then they both start paying them in Hamer Twp. about the same time after that. I'd say it's a pretty good bet they are brothers who married Jones sisters, don't you think? And if they came from Salem prior to 1850, then they may be tied in with the Davidsons in the book.

Also, there is a William Davidson in the 1840 census in Clay Twp. and that corroborates with the William S. in the tax rolls for Clay Twp. in 1840.

Don