Singers 400 series

The 401 was top of the series (and most expensive of the lot), with a built in camstack allowing for many stitch designs. Then there was the

403, a 'step down' from the 401 (and less expensive) mechanically the same but no built->I am curious about the 400s. How did the models in that series differ?

------------------------ Adult: A person who has stopped growing at both ends and is now growing = in the middle.

So what is the 401a?

Mavis