O is the 5th digit, so 2002, placing it in the "bad years" category of 2000-06. It's not that they were horrible or anything, but the speakers got darker due to a cone change by Eminence and they lost a lot of high end in comparison to years before or since. It will come back with EQ, but I don't think they sound as good. Some like the reduced highs, but I'm not one of them.
Ampeg Portaflex Club #1