PF-6 means Preamp Filter version 6. The control harness is what makes it a Series I or II, and it's not a detail that was recorded in the original records. These original records were only the information that would fit on a 3x5 index card, so the details are not very complete.
Over the years, we've transferred any card to a file when there was more information to be collected, like repairs, modifications, or ownership changes.