Would you like to be able to quickly and easily set your seam allowance on any machine? Well this is the perfect tool for you! Going from a mere 1/8″ up to 3/4″ in 1/8″ increments, and including a scant 1/4″ seam for quilters, you simply lower the tip of your needle through the hole for the desired seam allowance, then align a seam guide (sold separately) against the edge of the seam square, fix it in place and voila, no need to fouter about with rulers any more.
As seen in the photos this works with the Featherweight Shop Seam Guide (available as a bundle here) as well the vintage seam guide that may have come with your vintage machine. It would also work with magnetic seam guides, and on modern machines will work well with the Clover Stitch Guides.
Remember, if you change the size of your needle, you will need to re-measure each time, lining the needle up with the left side of the hole. It will work with needles up to and including a size 90/14.