I learned to sew at a Singer center in 1970, but bought a Kenmore because all the new machines at Singers had plastic gears, as well as the bobbins that wound in situ. I found that the bobbins didn't hold much thread and I feared that the hard plastic gear could easily get damaged. I'm sorry to admit that I have no idea what the model numbers were, but certainly the machines were made with plastic by the late 1960's.