The fact that it has a captive ac cord probably means it was a 600 that was upgraded to the R version when Frank designed it. Mine is the same, I sent it to him for the R upgrade. Later models have no captive ac cord-not sure when that occurred in the production cycle. There may also be updates when that change occurred. I wish Van Alstine would bring that model back though they will support it repair wise. Great amp! I want another to add to my collection.