Whilst other pre & post driver versions are available on the Linksys website, this version isn't, (as of Feb 2009), as also mentioned in several blog sites. And as far as I'm aware the product was only on the market for a short time as I tried to purchase one for a friend some months after buying my own.
The WPC55AG v1.2 only seems to work correctly with these drivers, a point even Linksys mention in the text files of the other versions. As to why they've failed to put this version on their website is a answer I'm still waiting on.
I hope this helps someone.