Walter D. Heinemann was a cousin of William Heinemann, the publisher.
He is referred to in JW's correspondence about 1896. A Walter Heinemann is listed as living at 35 Onslow Square in 1893.
It is unclear whether he is the same as the American born Walter Heinemann mentioned in the 1901 census as living at Ewhurst, Surrey. His occupation is listed as 'foreign baker' but this seems likely to be a mistranscription for 'foreign banker.'
Kelly's Post Office London Directory, London 1893; UK census 1901.