greywuuf,
That could be, or it could be for a second pin to fill bobbins off of or just there for dummies to ask questions about. Beats me.
I have a reprint manual for the 201 and I don't remember anything in it about dual needles. The 201 being a straight stitch only has the one tiny needle hole so it wouldn't work with dual needles anyway.
I have a number of machines that will work with dual needles though and they are really fun to play with. Especially those that will allow zig-zagging and decorative stitching with the dual needles.
Now here's a trick that makes a really attractive stitch; get two of the big serger thread stands with the tall hook on them and build you a thread widget. I swear I have a thread here about what I did, but I can't find it. So what I did was take a length of 3/8" dowel rod, drill a hole through the middle so it will sit on the spool pin and then drill two smaller holes crosswise at the end. I took the spring out of a clothes pin and put it in the holes at the end of the dowel rod, then put the two threads through it. From there I threaded my #2 66 treadle as if I were using one thread. As the two threads passed through the spring it twists them. When this gets sewed into a material it really looks neat.
Serger thread is thinner than normal thread but strong and flexible. I've yet to break it even once.
Here's a couple pics to ponder on: