This Shinwa Japanese square is a very accurate measuring instrument, designed for precision work. Here are some key features based on the description:

1. **Milled and black coated size markings**: This indicates that the size markings are engraved into the blade and then coated in black. This improves the legibility and durability of the markings.

2. **Completely Flat Blade**: The square has a completely flat blade, meaning it is ideal for measurements on flat surfaces. This ensures high accuracy when carrying out measurements.

3. **Stainless steel blade**: The blade of the square is made of stainless steel, which means that it is resistant to oxidation (rust formation). This is important to maintain the long-term accuracy of the instrument.

4. **Dimensions**: The square has two sides. The short side measures 50mm on one side and 75mm on the other side. The long side measures 100mm on one side and 150mm on the other side. The size indication is in millimeters and the accuracy is up to 0.2 millimeters on the longest side.

This high-quality square that is suitable for fine and precise work, such as in construction, woodworking, metalworking or other fields where precision is crucial.