Dude guess what... I did the same thing.
Ordered the Aftermarket Base because I thought that since my seat did not come with a facorty swivel that I would need the aftermarket base... OOPS.
Now I see that it is based on the type of Seat you have, not the type of base you have ... live and learn.
They are letting me return mine ... minus a 30% re-stocking fee. OUCH!
I was going to post that I would sell mine for 10% less than you would sell yours but I figured that would just be rude, so I will take the 30% re-stocking fee hit and you can keep the monopoly on incorrectly ordered swivel bases for sale.