Oops!! For some reason I misread the date as 1820. All I have said needs to be discarded. You are probably correct on the identity of Lewis Harter. Although I have no death date for him a birth in 1845 would have put him at 75 in 1920 - very possible.

Two possibilities for Mag Harter:
Martha Margaret Barnard (1883-1941), wife of Reuben Harter.
Margaret Hewitt (1844-1928), widow of Martin Harter.