Originally Posted by Jeff Wills

You're darn right. Perhaps Yellow Jersey has parts, but I'd try soaking it in penetrating oil first. And I mean really soak it. I would lay the frame on its side with the mounting stud facing down. Then use a couple layers of aluminum foil to mold a bowl around the stud, taped or zip-tied to the downtube. Basically, you want a container that will allow you to completely immerse both sides of that stud. Fill with your favorite penetrating oil and let sit a night or two. When you feel brave enough, remove the bowl, grip the "nut" with pliers or a Crescent wrench, and give the screw a twist. If it doesn't go one way, twist it the other- often all you need is break the corrosion up to get it going.

Good luck and Happy Thanksgiving!