Peerless question

When peerless made the model 500, did the hinged model switch to 501 or did it stay 301?

My understanding is that the hinged cuff went from Model 301 to Model 801 and that there were never any hinged cuffs designated 501. I know from archived Peerless webpages that the company was selling the 301 at the same time it was selling the Model 500.