Looks like someone on Cloudynights may have tracked down the original source!
In an 1873 issue of
Appletons' Journal titled "Eta Argus", Emma Converse summarizes the dispute about changes in the nebula and states
"In the middle of the brightest part of the nebulous light there was a dark vacancy, of a form resembling a keyhole, or the geometrical figure called a lemniscate, around which the light of the nebula was not uniform."
Later she mentions "
The southern loop of Herschel's lemniscate, or keyhole-shaped cavity had bulged out into the vacuity, forming an isthmus that trended north-south."
Sure sounds like she was translating Herschel's name into a simpler moniker. Here's the article --
http://tinyurl.com/26foe3u