The INK-0531 is the replacement blade for the INK 6 Siblings Keyhole saw (INK-0530), the small folding pull saw for cutting holes and openings.
It's the same 120 mm (4.7 in) blade the saw comes with, a thicker 0.9 mm in SK-85 carbon tool steel, for cutouts in plumbing and electrical work and for general cutting. When the teeth dull, fit a new blade and keep your handle.
INK saws are made by Ishinoko Kogyo in Sanjo, Niigata, Japan. The company started in 1946 as Ishida Saw Works, was incorporated as Ishinoko Kogyo in 1969, and designs and makes hand saws, including its Genbaya line.