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Out of curiosity, I took a look in Herschel's "Outlines of Astronomy" (1849), but although a page or so is devoted to Eta Car and the nebula, there's no mention of "Keyhole".
The reason I looked there is that in this book he uses the term "trifid" with regards to M20 ---
"One of the them [several nebulae in Sagittarius] is singularly trifid, consisting of three bright and irregularly formed nebulous masses, graduating away insensibly externally, but coming up to a great intensity of light at their interior edges, where they enclose and surround a sort of three-forked rift, or vacant area, abruptly and uncouthly crooked, and quite void of nebulous light. A bright triple star is situated precisely on the edge of one of these nebulous masses just where the interior vacancy forks out two channels. A fourth nebulous mass spreads like a fan or downy plume from a star at a little distance from the triple nebula."
Steve Gottlieb
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