The resistance/underground and Hogan's unit at Stalag 13 used "Frequency 510" to send Morse code via radio signals.
When Kinch is asked by Hogan if the same operator sent the code, he replies as far as he knows. Morse code radio operators were able to distinguish different operators by the way in which they sent Morse code. So he would have been able to at least give an educated guess.
Other prisoners mentioned, who are not in the regular cast, are Hammond, O'Brien, and Mills. Mills is used in one other episode. This is the only mention of Hammond and O'Brien in the series.
The opening theme is a bit faster than usual; it seems to be still in the same key as normal.
This is the last guest appearance of Horst Ebersberg in the series. In each instance, he played a German agent or enlisted man.