From a worn letter located in his pocket, ... (Para. 2)
Meaning: From a worn letter found in his pocket, ...
worn: a. old and torn because of repeated thumbing through and reading
located in his pocket: which was located (= found) in his pocket
Here the -ed verb phrase (participial phrase) is used as a post modifier and refers to "a worn letter".
In the last part of the same sentence we have "... his son was a Marine stationed in North Carolina". Here "stationed" plays the same role, meaning: "who was stationed ...".