According to a now-removed media advisory from 9/13:

The Smithsonian's National Air and Space Museum and the National Reconnaissance Office will be hosting a one-day-only viewing opportunity of the newly declassified HEXAGON (KH-9) satellite in the parking lot of the Steven F. Udvar-Hazy Center Saturday, Sept. 17. This is the first time the public will be able to view this impressive spacecraft, and it will be the only opportunity to see it in the Washington area for some time.

Read a little further down the discussion, and I can definitely confirm that there were MiG engines attached to tail fin of the shuttle, and the literature there identified it as the atmospheric test article.