According to ISMACS here -
http://ismacs.net/singer_sewing_mach...sses-1-99.html
"Central bobbin. One vertical shuttle, but fitted with two or more needles. The needle threads are interlocked by the shuttle thread forming two or more straight lines of stitching on the upper side, and an overseam (stay stitch) on the under side. 7 1/2" space at right of needle."
The number after the dash is VERY important in industrial machines. They can indicate completely different machines from another model in the same series.
According this listing of machines used by industry, these were used in a number of different capacities.
http://ismacs.net/singer_sewing_mach...-purposes.html