DIN 439 Hex Nut A4-040 Low

Material: acid-resistant stainless steel A4. Strength class A4-040. Metric thread pitch. A4 acid-resistant stainless steel, excellent corrosion resistance. Nut height: diameter x 0.5.

Special nuts are generally considered to be nuts whose design, application, dimensions, or other properties differ from a standard hex nut.
Examples of such nuts include low nuts (thin nuts), flange nuts, castle nuts, rivet nuts, square nuts, diamond nuts (diamond-shaped profile), sheet metal nuts, and welding nuts.
Materials include steel, plain or zinc-plated (bright zinc, Zn, zinc), stainless steel (A2), and acid-resistant stainless steel (A4).
Choosing the right nut? The rule is that the nut’s hardness, material, and coating should be at least equal to those of the fastener used (bolt, screw).