I cannot be of much help either! My book says simply 'Joseph, history unknown'. The book says Joseph had an older brother Heber b. 1769 in Salisbury, CT., and and older sister Sarah b. 1771, and that in 1771 the father Herber moved to Poutney, Vermont. I can only suggest that in Poutney maybe there is some record of his birth...???? You have more than I do!
Sorry I cannot be of more help,
We are cousins, love to share what I have with you on the rest of the Allen clan.