Singer 401a) built in zigzag and other decorative stitches
Singer 403a) needs a top hat to zigzag and do the decorative stitches
Singer 500) pictured on several pages - built in zigzag - pretty much the same internals - just has a different body than the 401
Singer 503) also needs a top hat to zigzag - same as 403 inside but has a different body - pictured above
Singer 404) slant needles - drop in bobbin - straight stitch only weighs about the same as the Singer 301
Singer 301) pictured on many pages above - very popular machine - has slant needles but straight stitch only. All but the Singer 301 have a drop in bobbin.
There are some other models of 400 series as well.
Singer 411G and a Singer 401G. (see page 6) They are ALL excellent machines - precision built for precision sewing.