DIN stands for ‘Deutsches Institut für Normung’ which means ‘German Institute of Standardization’. DIN develops norms and standards as a service to industry, to the state and to society as a whole. It is a registered non profit organization which has been based in Berlin since 1917. It consists of nearly 1,700 members who include individual companies, associations, public authorities, and other organizations from industry, commerce, the trades and research. DIN members support standardization through membership fees and play an active part in the decision making process.
DIN 985 defines the parameters for self locking nuts or "Prevailing torque type hexagon thin nuts with nonmetallic insert".
There are a variety of standards for nuts, bolts etc. that have been created to assure a level of quality for a given task. The standard will designate dimensions, chemical composition and mechanical properties. For a given standard (933) there are property classes (4.8, 5.6, 5.8, 6.8, 8.8, 9.8, 10.9 and 12.9) with different chemical compositions and mechanical properties. The N55 specification does not indicate which property class is required. Generally, the higher the property class number the "stronger" the bolt.
If your hardware provider is familiar with a different standard there are cross reference tables available on line that allow you to find equivalents in whatever standard is available to you (EN, ISO, ANSI etc.). I'd be comfortable with even a property class of 4.8 for this application. The key is to get something that has been rated as opposed to something made by a company that does not even bother to rate the hardware.