Compatible with models: OKI Microline 321 Turbo, 321 Turbo/n, 390 Turbo/n, 391 Turbo, 391 Turbo/n, 421n, 490, 491, 491n. Printers. This pull tractor includes the tractor assembly and a special, high-top access cover. The pull tractor is required for feeding continuous-form paper or labels from the bottom of the printer. You can also use the pull tractor together with the built-in push tractor for feeding paper in from the rear to provide more precise paper control.