WASHINGTON — For Paul Mellen, the trail began last March when he came across the Appleton Tracy pocket watch for sale on eBay. Manufactured in November 1862 by the Waltham Watch Co., it was solid gold, set with 15 jewels — a prized addition to his collection.

More treasured, as the Duxbury resident would soon discover, was the story behind the name engraved on the back, a Civil War officer named Major Jonathan Ladd of Lowell. The tale Mellen pieced together would lead to one of the most poignant moments in American history: the vigil at Abraham Lincoln’s deathbed.