Instead of using a foot control, some machines are fitted with a knee bar to control sewing speed. A knee bar is operated by the right knee pressing against the lever.
Singer electric portable machine knee bars come in two sizes: for full sized machines e.g. 15, 66, 201 and for the smaller three quarter sized machines, such as the Singer 28 or 99, where the knee bar will be shorter than the one used for full sized machines. The part number and the machines it fits is stamped onto the knee bar. The end of a Singer knee bar will have one of two types of fittings Singer Parts Album
Electric cabinet machines often have slightly different shaped knee bars. Early Jones knee bars for electric cabinet machines are made of wood instead of metal.
The idea of having a knee bar to operate a machine was first brought out soon after the use of electricity in homes became more common. There are some less common, early versions of knee bars and hand bars shown in Motors - Different Types
Other sewing machine companies giving a knee bar option include Bernina, Elna, Jones.