Flatbacker

Good flatbackers always fare well, which accounts for their swelling ranks. They work the offices along Madison and Lex like a Schrafft's super-hospitality wagon. Score the flustered john in the elevator, simple!

Source: wiktionary

If we had straight flatbackers, as we called them, they turned a date for twenty or twenty-five dollars. Nobody was allowed to rob them because they'd become regular customers.

Source: wiktionary

He had four girls in this stable and worked them as “flatbackers”—they would take anyone on, one after another, fifteen, twenty tricks a night, and it didn't matter if the girls were sick, menstruating, whatever: they had to work […]

Source: wiktionary