Pyrus communis 'Williams', 1/2" diameter at sale, sourced from New York.
Bartlett pears aren’t just popular in this country, they are also a favorite pear in Britain. But not by the same name. In England, Bartlett pear trees are called Williams pear trees and the fruit are called Williams pears. And according to Bartlett pear information, that name was given to the pears much earlier than Bartlett. After the pears were developed in England, the variety came into the control of a nurseryman named Williams. He sold it around Britain as the Williams pear. Sometime around 1800, several Williams trees were brought into the United States. A man named Bartlett propagated the trees and sold them as Bartlett pear trees. The fruit were called Bartlett pears and the name stuck, even when the error was discovered.
Read more at Gardening Know How: Bartlett Pear Information – How To Care For Bartlett Pear Tree https://www.gardeningknowhow.com/edible/fruits/pear/bartlett-pear-information.htm