"She was remembered even there among her former neighbors as a religious fanatic who had led her husband sternly and exultantly in the narrow path of enforced rectitude. When he was killed by a furnace explosion she declared it was the hand of God striking him down for some secret sin. Her associates were few: they were in the main members of a small congregation of East Side Anabaptists." (from The Greene Murder Case, by S. S. van Dine)
This is interesting, as I never heard of "Anabaptist" used as a specific denominational name in the United States. (It is a big country, and I could have missed it.) It was used that way in centuries past in Europe. However, more recently it has been used in a generic sense, such as "immersionist" might be attached to a certain group of people. The van Dine pseudonym was used by writer Willard Huntington Wright, who moved in artsy circles in the northeastern area of the United States, so it is possible that he was just guessing as he wrote the novel, without any direct knowledge of denominational usage.