This has come as a complete surprise: the image below shows that Yashica re-purposed the Contax Radio Control Set (on the right) to create the Yashica Wireless Mike Set (on the left) for their top movie camera, the Sound 50XL Macro.
I freely admit that I did not know any such wireless microphone set had been created by Yashica. You can see how they have used the external casings of the Contax set to produce it.
I found the set on a Japanese site and it's currently being auctioned. It must have been made in 1980 as it's not listed in the final edition of the User Manual. The conventional, wired Yashica DU-2 and more common CUL-3 microphones are illustrated; however, there is a Contax item shown as an accessory: the RTW Power Pack.
What I do not know is the operating frequency(ies) of the unit. The Contax Radio Control Set could not be sold in the USA because of its chosen frequencies although operation here in the UK was not a problem.