Analysis of information sources in references of the Wikipedia article "Radio-controlled aircraft" in English language version.
Use of frequency pins to identify the frequency in use. Pins, often clothespins, are marked with the color or channel number of the frequencies they represent. Only one pin is available at the flying site for each frequency. Transmitters shall not be operated without possession of a pin that identifies the frequency in use.
The FAA issued an Interim Final Rule today that will require drone pilots and model aircraft pilots to display their FAA-issued registration number on the outside surface of their aircraft. The rule took effect on Monday, February 25, 2019; which means the markings must be in place for any outdoor flight beginning on that date.