Many of the earliest radio episodes no longer exist. When the first episodes were broadcast, recording technology was still expensive and primitive by later standards. Audio tape was not in general use, and any recordings were made directly on to acetate discs. These could be played back, but tended to wear out quickly and did not survive unless further processed to create a master disc, which was only done for recordings intended for sale. Only one episode from the fourth series was preserved in the BBC Sound Archive (as a tape dub from an acetate disc).