The difference was the 128K was smaller at only 12-1/2" bed size.It always look a neater machine because of its compact size.
Its manufacturing year was 1955 to 1964 The Singer 66K is the large version of the Singer 99K.
Here the image shows a lovely 66K about 1920, originally may have been hand, electric or treadle operated.
The light aluminium frame was only 9-3/4" wide, even with the fabric rest down it still was only 14-1/2"....!
I'm sure its still the smallest adult sewing machine ever produced.
The Singer 99k sewing machine is small in size measuring only 12-1/2" wide, this image shows an early version the 99K (99K21 - 26) made between 1924 to 1954. The stitch length was adjusted via a screw and the machine did not stitch backwards.