Long before rotary cutters, this was an ingenious way for sewists to quickly cut bias strips with the perfect measurement for use with their Singer Binder Attachment. Simply insert the scissors in Bias Cutting Gauge slot, adjust the gauge to the measurement desired, insert the fabric in the gauge and cut. The fabric would remain in the gauge allowing you to cut a straight line, whether it be for facing, bias, etc. This particular accessory is a vintage Singer original. The ones marked with F, B & C on the gauge were for Singer Attachment sets, whereas the ones with lined measurements were more generic and commonly found amongst Greist Attachment sets. Their function is identical.
Using the cut raw fabric strips, you then insert them into the Singer Binder Attachment, which will produce the same effect as sewing double-fold 1/4-inch pre-folded bias tape. Perfect for single-layer fabric articles such as placemats, aprons, bibs, or enclosing other raw seam allowances. (Not to be used for quilt binding.)
This accessory is a must-have for Singer Attachment collectors and quite useful if you need bias tape in a pinch, and don't have access to a rotary cutter.