Japanese no.4 feather file for ryoba saws

A double sided file of a type typically used for sharpening ryoba saws. These can only be used on traditional oil quenched steel blades and not those impulse hardened (a sign being darker colouring to the teeth) which may stay sharp longer but cannot be sharpened once dull as the teeth are harder than the file. To use the saw blade should be clamped firmly in a vice and because the teeth alternate in direction the file should be pushed three or four times across every other tooth following the existing angle and a secondary bevel added to the top, before the saw is flipped horizontally and the procedure repeated (and again vertically for doubled edged saws). With a blade length of 10cm, and an overall length a little under 20cm.