D. W. Howland was an autograph collector.
Howland was living at a reputable Boston address in 1897. Whistler was not impressed by his terse request for a signature, and presumably never sent one.
However, he may have been related to Mary Bryant Howland, whose autograph collection was given to the Houghton Library, Harvard University. It dates from 1877-1893.
Houghton Library, Harvard University (MS Am 1812).