Oh, what a wonderful find!
I have a 401 and love it. Two things to know about the Singer 401 are:
1) Don't let it sit. It gets very cranky if not used for more than a week. Be sure to use the special stitch - the one needing the cam - at least once a week, too.
2) I am currently straight-line quilting a large and heavy fleece-backed quilt on it. The top is done on a foundation, and the batting is Warm and Natural, so you know that is one heavy quilt. The 401 is handling it with no problems whatsoever. Are you ready for a shock? I didn't even have to change from a zig zag foot to do it. I don't even need a walking foot on the machine!
What a great machine. It's a keeper.
It does take a bit of effort to figure out how to use it, but it is certainly worth it. The dial with the indicator on the left pushes in to set and the one with the indicator on the right pulls out. Be gentle. Use the fine setting and #3 width for special (cam) stitches.
I see you got a darner with it. I haven't tried free motion quilting or darning on mine. Let us know if you do.
Oh, there are two ways to thread it. I don't know the second one offhand but an internet search might provide it. And you can use two needles at the same time.
Enjoy!
Cricket