The orignal featherweight was actually the brain child of another company which if I remember correctly sold them under the name 'sewrite'.
The sales spiked every new semester around college dorms.
There was a flat black version for the military. And many unknowing house wives bought what a door to door sales man told them was the very last one.
During the late 40s thru 50s there was a trend of alternative materials for the body of full size machines making thm much lighterand more portable. Many manufactures got in on it.
The 301 is said to be the big sister of the featherweight. Some have a carrying handle, as well as not all have the same lenght extension bed.
The Singer 99 is a smaller version of the popular 66 (notice the number inversion)
The class 15 designs here given to Japan post WWII to help restart their delapidated economy. Singer launched suites. This was ironic because Isaac Singer the original founder of the empire was known to be a cheat, bully and a tiranical businessman who's only pursuit was wealth (and 5 wives). Singer tried to open a plant in Japan and was turned down for several years while they worked at getting their own factories up and running.
Both American and Japanese have their benifits. I tend to think the Japanese have better manuverabilty when I am making precision turns. The foot pedals also tend to be more fine turned so there are no jumps and starts. Both are fine machines not the mention the others out there. We work with mainly vintage machines and put them through a number of paces after cleaning and checking them end to end.