I picked up a Singer 403A made about the mid-fifties. Very clean condition; I'm thinking a 7 or 8. You can see it up there between the 401A and the 404 – right where it belongs.
It's a ZZ machine, and is essentially the twin of the 401, but instead of a cam-stack, it requires that a separate cam be installed for each decorative stitch. Many like this feature, as it allows for a simple switch of the cam for any given stitch rather than referring to the stitch chart under the "lid" of the 401, then setting the knobs accordingly.
This fairly completes my collection of the best of Singer's Slant-O-Matics from the "all-steel" era. I'm missing the 503(A).
Let's run through those I have then: The 301 was the first of series with an all-new design and all-aluminum construction; then, in the 400 series: the 401A, 403A and 404. Rounding things out is the 500A "Rocketeer".