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Yes, Monica is right - 15-91 is a potted motor, but the 15-89 is a HC. Here are the Class 15 machines.
15-30 very early treadle - early 1900's, 1895 - 1933
15-90 Electric/external motor
15-91 Electric/potted motor gear driven
15-96 Treadle 1940's +
15-98 HC 1940's +
15-125 Electric after 1950
http://www.quiltingboard.com/t-133081-1.htm bag 4 school
http://www.quiltyard.com/whatiscottontheory.phtml COTTON THEORY
http://www.quiltingboard.com/t-26051-1.htm Turkey tracks
Don't be scared you have the board to help you. I have not done one, but I would use Cutbuns help on the tutorials. She called them hexagons, I think that is the same as OBW.
http://www.quiltingboard.com/t-132848-1.htm checks, leaves
How to use a treadle AND how to use a Singer 66: http://www.youtube.com/user/TreadleL.../6/Fi8a_OHUv1M