Q: Did the Singer 66 model/style evolve/change over the years???
A: Yes
I acquired one at a church sale a couple of years ago - and I think it is a 66-16.
Q: What do the -xx mean?
A: The last two numerals are variations of the model. The numbers can ID the machine as a treadle, or a hand crank, or an electric or other things too.
When I looked at the listings, they were 66-16, 66-18, etc etc.
Q: Did the basic style/guts stay the same, or did those change over the years? I think the decals changed over the years.
A: The basic internal design stayed the same. The variations were mostly external and had to do with the controls and functioning of the machine. There were quite a few different decals.
Joe