Whether it was given a designating letter I do not know. There were four different models of the featherweight the 221-1, 221K4, 221K5,and 221K7. That is the only model of FW that Singer made with the switch on the light housing, all the others have the switch on the bed of the machine.
Another check you can make to determine which model you have is to check the model number of the motor (see the plate mounted on the motor).
Finally, its sleekness, elegance, and mechanical simplicity appeals to the design sense of women and men who appreciate the beauty of color, line, and texture in their quilts. While many existing machines were well-loved and thus well-used, Featherweights are by no means rare.For modern seamstresses, it's too basic, sewing only a straight stitch without even a reverse setting on early models.Even during the years of its manufacture it was sometimes ignored by the sewing public, as looking too much like a toy, and being too expensive for depression-era and wartime pocketbooks.Some "mint green" machines are also rumored to have been made, but opinions vary over whether this was really a green machine or merely a white one with a green tinge to the paint.Larry Oliver, a Featherweight collector on Compuserve, wrote to me: "I have seen a tan machine and a mint green machine (definitely NOT white). The rarest variant I have seen belonged to an old fellow who owned a Singer store in a small town for 50 years.The Singer Featherweight portable sewing machine is a model made by that company between 19.