I'm not sure about the #s in the 6mm but a 6mm in the Model Six is a LOW production unit.
IMO - if you like the 6mm R and you like the condition of that gun.... I'd give $500. pretty quick. Pawn Shop ? Haggle, Haggle, Haggle.
Jerry
ps - Search the Deer Hunting Forum, "Pump Rifles and Deer" . 2014-2015- There was a long thread - many pages - There were many participants beside myself. MagMarc, SKane, Whelen Nut, et.al. There was a list from the manuf of Rem 760s by caliber & Model Sixs by caliber.