On page 357 of "The Manning Family", William Cutler wrote that he received his proof that Horace was the son of Jacob by corresponding with then living brothers and sisters, but that the account was "imperfect" because some had not replied to his requests for information.
He lists Horace as the oldest, born in 1803, Apr. 11, in Vermont.
I am descended from Horace and his second wife, Betsey Kelley, of Allegany County.
I have copies of the Cutler book pages from the LA City library collection. I would very much like to learn more about Betsey Kelley, if you have any more information.