Hartshorn and Slaughter is a prestigious litigation boutique firm in New York City. Richard P. Woolsey was an appellate advocate for this firm before he joined the United States Army Corps of Engineers. (SGA: "Inquisition")
Salt of hartshorn was a popular smelling salt in the seventeenth and eighteenth centuries, often used to revive ladies who were suffering from 'the vapors'.
The similarity of "Hartshorn and Slaughter" to evil law firm "Wolfram and Hart" from the Buffyverse suggests an intentional homage by the authors.