I believe it's real, just a little bit of fun.
The Wikipedia article on the SCA states it as fact, as do other historical articles.
Search engines will lead you to multiple pictures of the attach points from different angles, all showing the label, for example:
It looks like an earlier version of the label, since painted over and replaced, was even sillier:
"Lefty Loosy, Righty Tighty" being a well-known mnemonic for which way to turn standard-threaded mechanical screws and bolts.
Here are some pictures I took on April 29, 2014 as the Shuttle Carrier Aircraft (N905NA) was being moved from Ellington Field to its final resting place at Space Center Houston.
You can see the label as I zoom in (and walk down the railroad tracks) in the series of pictures, especially if you view them full size.
It's real. I took the original photo of the message that you posted, at the Edwards AFB open day. I have the original image from which this crop was taken (right down to the lens flare over the fastener!). Good humor!
Original photo from which the crop was taken to Wikipedia. If you look closely you will see the lens flare over the fastener, same as in the crop. Lighting direction and quality exactly the same etc.
Also, here's the wikipedia discussion, similar to what you are having here . . .