They fit into the Futaba transmitter and replace the 40MHz radio with a 2.4GHz one.
I've been spending some time playing around inside a 6EX 2.4GHz transmitter, getting my eight way switch working and realised I still had a couple of the 40MHz transmitters, so I've bought an X8D module and an X8R7-L 7 channel receiver, so I'll post a quick "How-To" once they arrive.
Adrian.