Singer purchased W&W in 1905, and continued to sell W&W D-9s with the W&W decorations until about 1908. After that Singer sold the D-9 with some modifications as the Singer 9W.
I've been keeping track of any dated W&W receipts or warranties that have serial numbers. I have a picture of a D-9 warranty serial #2749313 dated 02/03/1904, so I would say it is about 1904.
Tammy Tatum