My Mom has a Singer 401 hidden away in a cabinet and sews on a 70s Kenmore that sits on top. When knits came in the 401 didn't do them well and my Dad went and got her the Kenmore, it is a basic model that does not take cams.
I've checked it out and the stitch selector is frozen (no surprise, since I remember the can of 3-in-1 oil that used to sit on the windowsill, it's now hardened in that machine). It's promised to me so someday it will get restored.
The 401 has the built-in camstack and does many of the stitches that would require a cam on the 403. It does take black top hat cams too, for additional stitches (I think #8, 11, 15, 18, 19 are the cam that aren't built in? I've seen a chart somewhere). Otherwise, the 401 and 500, and the 403 and 503 are mostly the same inside. It is a slant shank, that came in with the 301 and continued through the Touch & Sew line. You can buy accessory sets for T&Ss a lot cheaper than you can can for a 400/500 line and they are mostly the same although the T&Ss may come with chain stitch accessories that the 400/500 can't do.