Here is just a little note I want to make myself on how by adjusting the red lever you might get variations of the same stitch. You can play with more options.
For example, the BN 5 combination produces the “Pyramid Stitch” in your Singer 401. It is a built-in stitch so there is no need to use a special or additional disc. By moving the red lever to various numbers, you will get different looks. So you can feel free to experiment and see if you find these interesting and would like to use them in future projects. Lots of stitches to choose from if you think about all of the possibilities.
The first try was really ugly and it was because the thumb nut was completely tightened against the metal plate on the “Fine” position. You want to move the lever at the “Fine” position to produce decorative stitches but you must avoid having the notch touch the metal plate. This will produce nasty tight stitches that will break needles, and give you a lot of grief!