DIN 439 MF hex nut A4-040 fine thread

Material stainless steel A4. Strength class A4-040. Metric fine thread (MF). Acid-resistant stainless steel A4, excellent corrosion resistance.

Special nuts are nuts whose design, application, dimensions, or other characteristics differ from a standard hex nut.
Examples of such nuts include thin nuts (jam nuts), flange nuts, castle nuts, rivet nuts, square nuts, diamond nuts (diamond-shaped profile), sheet metal nuts, and welding nuts.
Materials include uncoated or zinc-plated steel (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).