That's was a popular accessory manifold sold over the counter at Ford dealers. Bruce Lancaster is correct that it is for dual exhaust set up. Worth $50-$100 on ebay, but I've even seen it sell for as much as $300.
The manifold you want is the standard one that exits forward of the box that can also be used for dual exhaust. They so common that they hardly sell at all. You can pick one up at a swap meet for $10 or less, same on ebay --around $10. Don't bother with $300 Fenton, you may run into more trouble.