This is a very good set of carbon steel feeler gauges made by Shinwa in Japan. There are 25 leaves covering the range from 0.03 mm ( about 0.0012") up to 1.00 mm (about 0.039"). The leaves are 100 mm long which is long enough to make it easy to combine two or more leaves to create an intermediate thickness.
Feeler gauges are very useful for various tasks in the workshop including use with a straight edge to determine how flat a surface is and also creating precise offsets when setting up fences, planer knives, increasing router plunge cuts etc.
These thicknesses and accuracy of the leaves in this set are as follows:
Accuracy is ±0.005 mm (about 0.0002") for leaves between 0.03-0.15mm thickness, ±0.012 mm (about 0.00047") for leaves between 0.2-0.5mm thickness, and ±0.020mm (about 0.00078") for leaves between 0.60-1.00mm thickness.