Hi All;
I will try to combine several answers in this one post. The majority of #8 machines had the larger 1.0" (25.4mm) dia? bobbin by 0.245" (6.22mm) thick. Its case did not have a tension spring, tension was applied externally to the case. Late in the #8 production the bobbin was redesigned to a bobbin of 0.866" dia (22mm) by 0.278" (7.06mm) thick. The bobbin case had a built-in tension spring, but did not have a locating finger. The bobbin holder (Drop) was also redesigned & these normally cary an?1885 patent date.? Very early? #9 machines used the identical bobbin & case. Very shortly a locating finger was added to the case which also required a clearance cut be made in the bobbin drop ring.?
The D-9 changed to a built in bobbin case with a latch finger & did away with the bobbin drop. Its bobbin retained the same diameter but was thickened to 0.305" (7.75mm) for a slightly greater thread capacity. The D-9 bobbins have a single hole in each side down close to the hub for identification, #9 bobbin sides are totally smooth. Singer 9W machines used the same identical case & bobbin until late in their production when the case & latch finger were slightly re-shaped to allow for the flat sided bobbin. This has nothing to do with whether it is a 9W1 or 9W7. So far as I am aware all the ones with the flat sided bobbins are 9W7 machines "UNLESS" the bobbin case has been switched from a later machine.
Singer 9W machines will be found with both the 7-digit SN as well as the shorter number with the W addendum. So far as I know the reason for both styles of numbers has never been determined. Both styles of numbers are found in both the 9W1 & 9W7 types. There are other sub-models other than the 1 & 7 but these have to do with slight variations, such as fitted for a hand crank, equipped with a hand crank & furnished with a knee lifter for the presser bar & etc, but they will all be of the basic 1 or 7 configuration.? The 9W1 has W&W decals & fits W&W wood, while the 9W7 has Singer decals & fits Singer wood.
The #9 is a totally different machine than the #8, I do not believe there is a Transition as such. I do have a very early #9 which uses the bobbin case without locating finger, same as late #8 machines. Its SN is 21,041 & it does not carry the March 25, 1890 patent date seen on the majority of #9 machines. Other than the bobbin area it is strictly a #9 & nothing like a #8. I would not call it a transition machine, but rather a New Model.
My Singer?9W7 incidentally?carries a SN of 3,185,801Applying "The Formula" to it would show a build date of 1908. As it uses the flat sided bobbin I feel it highly unlikely it was built prior to 1910 at the earliest. It seems total production of these machines had come to an end by 1913. They were replaced by the Singer model 115-1 which was introduced in 1912 & also a rotary, based on the W&W design but revamped to use ase a bobbin case similar to the Singer 15s. It also very closely resembled the Singer 15-30 in appearance.
If I skipped anything feel free to ask again.
Miller/TN.
