Time to check in
If I'm not mistaken, the PWA and PWA-II look about the same, and have the same controls... so it could be as old as 1992, or as new as 2005.
I don't know if either one sounds decent on bass, though, as I've used few of their early-90's models.
If you can ask the seller to open it up (only four screws need to be removed - they're all on the sides of the pedal), there will be a date and model printed on the PCB, usually in the corner of it somewhere. It'll be easy to spot.