My grandfather worked at Lockheed when the shuttle was first being built, they did a lot with the launch systems. This orbiter was used for approach and landing tests. No one had seen these pictures before and as a 5 year old I was really mad when people told me it was a model!
The Constitution was later re-named Enterprise (thanks to Trekkies) but it was still Constitution when these photos were taken (75 I think)
** NOTE - I was corrected, this photo is actually a full-scale mockup in Downey, California, from February 27, 1975. Sorry for the error, but I got these photos when I was 5 and the story clearly got a bit distorted.