Those are fun to play with - several companies made them and they're especially handy if you have a model for which no darning foot was made.
That's so cool that you got the hoop and directions with the spring - most of mine were loose in old treadle drawers or mixed in with a box of attachments, the hoop and directions long gone. If you didn't know what it was, you'd think it was just a piece of bent wire - someone messing around with a paper clip. :)
Someone on one of the mailing lists actually made a spring that works like this out of a large paper clip - she had a back-clamping 66, so no darning foot. This seemed to work for her. :)