The following is assembled from a thread in sci.space.shuttle regarding the origin and differences in the Shuttle Carrier Aircraft (SCA), which are the two 747's NASA uses to move the shuttles, piggy-back fashion, about the countryside.....
In response to the original question, Ben Hallert wrote:
>"The first SCA (from American Airlines) purchased (N905NA) was used for
>the ferry tests and free-flight tests in the 70's. It remains in use,
>and was joined in 1990 by N911NA, a 747-100SR from Japan Airlines."
Michael Morton added:
>"Also 911 has a small notice on the main orbiter attachment point >saying, "Attach Orbiter Here, black side down."
And then Steve Wachowski added:
>It also says, "righty - tighty, lefty - loosey"