So Fine Prewound Bobbins. Cardboard-Sided M Style. Dozen Pack. 138 yards per bobbin.
Cardboard  M style with So Fine! thread. 50 wt. lint-free, extra smooth polyester.

Cardboard-sided prewound bobbins may be used with the cardboard sides on or with the sides removed.  Please experiment to determine what works best in your machine.

Below is a partial list of machines compatible with the M style bobbin:

A-1
Gammill
Handi Quilter - Sweet Sixteen
Homesteader
some Noltings
Tin Lizzie 18
Innova
Baby Lock Crown Jewel
Viking MQ 18X8
Pfaff GQ 18x8