If you put the thread through the check spring before the bar, it will bend the check spring under bar and possibly break. Notice the distance and shape of the check spring at
Threading a Singer 15-91 ? Also note this is a 15-80 not a 15-91 as they are threaded differently from each other.
The image at the link is like the Federal!
Janey - Neat people never make the exciting discoveries I do.