I guess you didn't quite get the gist of my last post.
The spread you are talking about is the spread between the 30Y futures and the Ultra futures contracts. Both contracts track bonds that are less than 30 years in duration. The U.S. does not issue 50Y bonds, so the Ultra has nothing whatsoever to do with a 50Y bond. In cash terms, the BOB represents a spread between a 15Y bond (the current CTD to the Bond futures) and a 25Y bond (the current CTD to the Ultra futures). Hope I made this clear.
If you want to learn about the relationship between fixed income futures and cash bonds read Burghardt's Treasury Bond Basis book.