[ARC5] PL-55 and PJ-068 type plugs with retaining rings inquiry

[Jay Coward]

Hi Folks,
 Ever notice on ARC-5 gear that key and mic jacks have extended threads on the outer sleeve? I have two plugs that fit the key jacks on several of the transmitter control boxes and MD-7 and can be secured with a threaded retaining ring.


 One is marked B180207 and has a black plastic shell. The other is marked 275 and appears to be able to be soldered directly to a properly prepared shielded cable.


Can anyone elaborate on this? Obviously someone thought it important that these did not get inadvertently unplugged. So why are these uncommon?

Thanks,
              Jay KE6PPF