Try this link for Bill Holman's manual for the HA-1 class 15 straight stitch sewing machine. The Singer 15-88 through 15-91 has a different bobbin case as 'finger' on it points to the 11:00 position. The HA-1 has a 'finger' on the bobbin case that points to the 1:00 position. However, the manuals are mostly the same.
http://www.ismacs.net/singer_sewing_...ion-manual.pdf
Cathy