I have referred to this recently published alleged manuscript by Cherisey before and have learned that the phrase - UN VEAU À CINQ PATTES - means something hard to find.
At languefrancaise.net ["a dictionary of slang, & popular colloquial French"] it says the phrase UN VEAU À CINQ PATTES means "something hard to find" and it's synonyms are rare, infrequent, seldom. [See https://www.languefrancaise .net/Bob/58703].
A further definition is 'something out of the ordinary'.
This particular meaning was popularised by the eponymous film by Henri Verneuil, and its expression designates something that is a rarity.
One is to understand thus; a Sheep or Calf would only have five legs in the case of a genetic anomaly, and therefore an animal such as this would be a real rare pearl or, in the case of a person, an exceptional human being.
Cherisey is very funny because the 'calf [or sheep] with five legs' according to the second definition would be Henri Boudet, often noted to be a rare and intelligent human, indeed an exceptional person. But in the first definition the five legged calf would be the Cromlech at Rennes-les-Bains - which most definitely is something hard to find because it does not exist.
Cherisey's manuscript has the tag line - notes sur l'oeuvre de l'abbé H. Boudet, sur ses sources, son tempérament, son idéal et sa vie [notes on the work of Abbé H. Boudet, on his sources, his temperament, his ideal and his life.].
So the title of the manuscript describes the rare and exceptional priest Henri Boudet and no doubt his peculiar work 'La Vrai Langue Celtique et le Cromlech de Rennes-les-Bains'. It also describes the Cromlech, the subject of said peculiar work, which is hard to find, because it is an invention by Boudet.
I guess the notes on Cherisey's sources for Boudet and his life is meant to help you work out why Boudet invented a Cromlech around the village of Rennes-les-Bains and what this has to do with a Celtic language. He wrote in STONE & PAPER;
"... the[re] is a difference in ... phenomena which should be seen to be believed, & the Cromleck of Rennes-les -Bains [which] is seen only when one believes in it ...”