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For those interested in PLG's history, the Brooklyn Historical Society has a fascinating digital exhibit about the Lefferts family:
<http://www.brooklynhistory.org/exhibitions/lefferts/>
The exibit ends with the sale of the Lefferts' farm for development, but there is this reference to the single family covenants:
"John Lefferts (1826-1893) broke up and auctioned 516 parcels of land of the family’s Flatbush homestead. “The absurdity of devoting lands so desirable, central, and valuable to raising corn and potatoes,” the auctioneer of Lefferts’s land announced, “has finally induced the owner to part with [the] lots.”
John’s son James oversaw further development after his father’s death in 1893. Though the family sought to profit from the area’s real estate boom, they also hoped to shape Flatbush’s development on their own terms. James laid down a restrictive covenant limiting commercial or industrial development and requiring all dwellings built on former Lefferts land to be single family residential homes".