This is a nylon washer for CO2 regulators. These are for regular re-use, but can sometimes deform after prolonged use and need replacing – or maybe you just dropped it somewhere! With many regulators, the tip that connects to the CO2 cylinder can be unscrewed and the washer replaced before re-screwing the connecting tip. This ensures that the washer is secure and will not come loose.
For temporary disposable washers, check our hard fiber regulator washers.