I don't know when Singer introduced the smaller solid hand wheel. There are a few places on the web naming model 99 as the first machine factory fitted with motor and lights, early 1920s, (21 or 22). Motor was available decades earlier, but were either fitted on later or on special order. I have yet to find comprehensive info on this. The prices on motors dropped around 1930 making it much more affordable to convert older models to motor and lights. It was commonly done way into the 1950s, probably until around 1960. We still fix up these old straight stitchers for their dependability.
I have a 99 and two 201s, and no need for another straight stitchers. That said, both of these are nice fix up machines, and someone will be intersted in them. I value both of them about the same. For the 1950 machine, a missing bobbin cover is easily replaced. Cabinets and cases are harder to find, but motor, lights and pedal are there and are a pluss. A pedal easily go for $10-20? I payed just over $80 for my 201 in a small cabinet, a few smaller parts needed to be replaced, but accessoires were there and mostly complete. I guess fixups like these will go for $30-40 to a person looking for a straight stitcher?
I have read of at at least two people who had looked for more than three years to find a "red eye" 66, and jumped at the first chance. I don't know how common it is, but it turnes up now and then just about anywhere.