It costed a lot but I have seen adverts for such hidden radio in magician's magazines from the time. There has always been a very lucrativ market for an engine

It costed a lot but I have seen adverts for such hidden radio in magician's magazines from the time. There has always been a very lucrativ market for an engineer+magician that could provide hidden communication devices at the fringe of technology. One of the best and oldest device I found used a telegraph (!) hidden in a turban and shoes with copper sole on a copper threaded carpet.