Singer model 15 - "Grandma's" - 1935 and the 99K - "the baby" - 1954
This was the machine my Grandmother sewed on for all the years I can remember. It is an electric and is the only model with tension on the left.
This is the 99K and looks similar to the 15. It is an electric and is much smaller than the 15, or 99. It is actually 3/4 size of the full sized 66 and 15 machines.
Here you can see the difference between the 15 on the left and the 99K on the right. Both are very plain machines.