I came across this interesting part at the Tokyo Radio Fair last month.
It is a BNC strain relief for the popular Diamond RHM8B antenna, particularly when used with a PL259 socket adaptor on an FT-817. If anyone has one of these antennas, you will know how fragile it is at the BNC plug end.
The strain relief is simply a small plastic tube that provides support to the main shaft of the RHM8B which is then supported by the outer of the PL-259 socket on the FT-817 rear antenna.
This is the coupler or strain relief, not sure what to call it!
The tube was 500 Yen (~ $6 AUD) at the Radio Fair, the supplier details are on this web link.
It provides some piece of mind in case the antenna is knocked or catches on something. Of course it will only provide limited protection but it is certainly better than no protection at all. It might be possible to make one using an electrical conduit coupler from a hardware store, although this unit is a reasonably tight fit at both ends.
Note that I have two PL-259 male to BNC female adaptors and only one was a good fit on the rotating threaded section inside the tube.
The strain relief tube on the back of the FT-817.