Okay there has been a lot of talk about these machines and I thought I would shed some light on the matter since I own one of each now. I will include photos of them so you can compare the two together and then you will see the differences.
Singer bought out the Wheeler and Wilson factory in 1905 and after they took over they grabbed the left over parts from the D9 and decaled them out and sold them as Singers. Then after they cleaned out the surplus they started casting their own versions of the machine and selling them for a short time after that.
We have already established that the machines that were made from the left over parts from the W&W's had a "W" in front of the serial number and the later Singer cast ones had the plain old numbers but still used the Wheeler and Wilson serials to keep up with the production. But there are several differences between the two machines that you can not tell unless you have them sitting side by side.
The biggest difference between the two you can see is the fact that the Singer machine has a longer and wider base to it. The engineers were trying to keep everything uniform and semi interchangeable by adding to the bed so that the machine would fit into their cabinets with out having to retool for the machine. The spool pin was in an entirely different place and was just a pin and not the fold over spool holder that W&W was famous for. In fact the casting is completely different because of the bed extension and the serial boss in the rear of the machine. Even the hinge pin locations are in different spots and the Singer machines had grub screws to hold them on the pins where the W&W relied upon a drip pan that was hinged into the cabinet to hold the machine in place.
The things that are almost verbatim are the bobbin assemblies on the early Singer castings. They have the same exact parts and part numbers. The face plates are identical including part numbers on the upper tension and the mechanics behind the faceplate are the same.
W&W D9 on the left, 9W on the right
Spool pin for the W&W
Spool pin for the Singer cast machine
Bobbin assembly for the Singer cast machine (early version)
Veiws from the front W&W still on the left. The Singer is wider than the W&W
W&W Bobbin assembly
Bobbin winder for the Singer cast machine
Difference in the length
Underside of the machines. Notice the difference in the hinge pin locations W&W is on the left.
Bobbin winder for the W&W
Veiws from the rear W&W still on the left