The 301 was Singer's first slant needle machine. Its design was also the first sign that Singer was moving away from the Classic look.
The 301A is the same model as the 301 - it was just built in a different factory. The 301A was constructed in the Anderson, South Carolina factory.
The 401 was a the first Singer that could sew decorative stitches by changing the settings on the machine.
The 500 was a capable machine that could sew a wide range of decorative stitches. Its most notable feature is its "space age" inspired design.