Information from: “Martha's Vineyard Sign Language” (607-627) . Joan Cottle Poole Nash (2015) , Julie Bakken Jepsen and Goedele De Clerck and Sam Lutalo-Kiingi and William B. McGregor · Walter de Gruyter
"...perhaps as many as several hundred at times in the past."
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"The language fell out of use with the death of the last signing deaf resident of the town of Chilmark in 1952." "In 1950, Eva West Look, the last of the Chilmark hereditary deaf, died. Katie West, her sister-in-law, whom Eva had brought to the Vineyard, died in 1952. Though some hearing Chilmarkers continued to use the sign language among themselves, they did not use it with people in the town who became deaf, nor with people from out of town."
Martha's Vineyard (island off the coast of Massachusetts), late 1600s-1952
Information from: “Personal Communication on sign languages” . James Woodward (2012)
80 percent certain, based on the evidence available
Only a few elderly users (all probably over 70) in 1985