Archaeologists have pulled a 400-year-old slate tablet from what they think was an original well at Jamestown, a historic preservation group announced yesterday. The slate is covered with faint inscriptions of local birds, flowers, a tree and caricatures of men, along with letters and numbers, according to Preservation Virginia, which jointly operates the dig site with the National Park Service. It was found May 11 at the center of James Fort, which was established in 1607 and makes Jamestown the first permanent English settlement in North America.