This was something I used to wonder about - the differences between Singer 401, 403, and 404
|Singer 404 (straight stitch)|
|Singer 403 "Semi-Automatic"|
|Singer 401 "Automatic"|
The difference basically comes down to this. The 403 is "semi-automatic" because it zigzags only with a cam in place (it is shown here with the "0" cam that makes the zz stitch.) Other decorative stitches can be made by popping this cam out and substituting one of the other 22 top hat cams that are available. This is pretty simple, and straightforward - choose your stitch pattern and pop it in. You choose your stitch width and needle position as usual.
|Singer 403 takes one cam at a time|
Then you have the 401. It is considered "automatic" because the zigzag stitch is built-in via a camstack. This also includes a number of other fashion stitches which you arrive at by a somewhat complicated system of turning two lettered dials to get your stitch pattern. To add to the confusion, it also takes top hat cams to make different "combination" stitches. (I explain that in a little more detail here.)
|Singer 401 with camstack, plus takes extra cam for "combination" stitches|
In all other respects, these machines are identical. They both have horizontal (drop-in) bobbin with rotary hook, dual spool pins and dual upper tension for twin needle stitching, three needle positions, the elevated throat plate system, bobbin winder on the side, and a flip top for cam placement.
(NOTE: the difference between the 500 and 503 Rocketeers is basically the same as the 401 and 403 - one has a built-in camstack and the other requires a cam to zigzag .)