Here's something - apparently Herschel first sketched it in 1830:
"The enigma of the dark keyhole structure dates back to 1830 and the first detailed drawings of the Keyhole nebula by Herschel, which show a keyhole shape that is somewhat different from today's picture. After much controversy about the reliability of Herschel's work, it is now generally accepted that the variations of the keyhole shape are real and were associated with the large luminosity fading of n Car after its bright phase from around 1822 to 1856"
http://iopscience.iop.org/0004-637X/...2740.text.html
Cheers -