DUXBURY — What experts are calling “the rarest of the rare” and “a once in a lifetime find” — a largely intact woodworking shop dating from the latter half of the 18th century — has been discovered in Duxbury on the site of a private school for children.

“It is an extraordinary find,” said professor J. Ritchie Garrison, a specialist in American material culture who hurried from the University of Delaware to take a look at the shop last month when he heard about the find. “It’s National Historic Landmark status.”