Thanks for the tip.Unfortunately I already ruled out Isaac b. 1774; he died a bachelor in 1805.There's not much chance of a mixup because he was listed in the Newbury death records as "son of Enos & Sarah, aged 31 years 27 days."There's no mention of a widow.
The Massachusetts VR states that the Isaac I'm looking for was born in Newbury, but the only Isaac born in Newbury at a plausible time was Enos & Sarah's son.This leads me to wonder if the MA VR is wrong or if the birth was not recorded locally. Isaac and Rebecca (Sargent) Bartlett had four children listed in the Newbury Vital Records, but I found references to two earlier children who might have been born elsewhere, one of whom was named Isaac.
Another reason I thought this lead looked promising is that Rebecca was born in Haverhill and had a son named Benjamin.I disovered a bible in my great-aunt's attic inscribed "Benjamin Bartlet, Haverhill 1848."I don't know how he was related to us.