For the lowest tech clothes pin frequency pins, simply write the frequency on one end of the clothes pin. Then it can be either on the board - when the frequency isn't being used - or clipped on to the Tx handle when the frequency is being used. If the frequency pin of your radio isn't on the board it means that you should not turn on your Tx as that frequency is being used already. A step above that is to make a square plate and glue this on to one end of the pin and on this plate have the frequency number printed, etc.