Things new campers should know about the gas supply in caravans and motorcaravans
Connection screws to gas cylinders with flammable content always have left-hand threads. This different turning direction ensures that no mistakes can be made.
The left-hand thread screw connections, e.g. hex nuts, are always marked with a notch (see figure links). This is standardised and anchored in the DIN EN 15202 as follows for liquid gas cylinders: “The corners of the hex nuts with left-hand threads must bear a notch for better recognition (e.g. a 60°-V notch).“
This picture shows one of the more rare left-hand thread wing nuts with notches.
In contrast, left-hand thread wing nuts do not always bear a notch, because this is not stipulated in the standard.