Your guess is correct.
Go to https://www.geoportail.gouv.fr/carte with layer Carte de l'état-major, put your finger on Crêt de la Neige 1723 m (corresponds today to 1720 m as the altitude of Pierre du Niton is now 3.04 meters lower than in 1828) and change then to the layer carte topographique IGN: your finger is now on the point 1720. J1 is a fake discovery of this known point.
Look at
https://www.letemps.ch/sciences/le-pic-anonyme-devenu-toit-du-massif-du-jura-n-est-pas-si-anonyme-que-cela
for the correct story. By adding the name J1, you perpetuate the fake.