The OKI ML320 Turbo and ML321 Turbo are 9-pin dot matrix printers while the OKI ML390 Turbo and 391 Turbo are 24-pin dot matrix printers.
Get the spec sheet for more details.
ML320 Turbo P/N: 62411601
ML321 Turbo P/N: 62411701
ML320 Turbo/n P/N: 62415401
ML321 Turbo/n P/N: 62415501
ML320 Turbo/D3 P/N: 62412901
ML321 Turbo/D3 P/N: 62413001
ML320 Turbo with Cut Sheet Feeder (Single Bin) P/N: 62411603
ML321 Turbo with Cut Sheet Feeder (Single Bin) P/N: 62411703
ML390 Turbo P/N: 62411901
ML391 Turbo P/N: 62412001
ML390 Turbo/n P/N: 62415901
ML391 Turbo/n P/N: 62416001
ML390 Turbo with Cut Sheet Feeder (Single Bin) P/N: 62411903
ML391 Turbo with Cut Sheet Feeder (Single Bin) P/N: 62412003