Here is a Facebook exchange I had recently with Ernie de la Fuente:
Ernie de la Fuente:
Dont know if anyone is tracking this stuff any more but I found that at least one version of OPC 2nd series original uncut sheet code is 4-427-05-04-3. Doesn't that imply it was a 4th version of the sheet and it was from 1973? I know there was some speculation at one point about OPC being 1974 and later.
Dave Schmidt:
Wow. Pretty sure this is unknown. Pretty confusing too, because the 4th series OPC sheet is uncoded. The code you describe seems like it should be the 4th re-setting of the domestic 2nd series sheet. The only thing I can conclude is that for these early OPC series, the final version of the domestic sheet was used to set up the OPC sheet, and was coded accordingly. That would certainly suggest the OPC came last. It may still have been released in 1974 in theory. At some point they must have stopped coding the OPC sheets.