SCR-300 / BC-1000: Question about "Automatic relay Equipment"?

[SGM (ret.)]

Ok, so I'm doing a bit of a deep dive on the SCR-300 Radio Set, and one of the first questions that I have is about the use of the "jack for automatic relay equipment."  I have searched TM 11-242 for any information on the equipment that was intended to use this feature without any luck.

 

 

There is only a single statement in the TM under the section on the controls for the radio, and it simply states, "i. RELAY Jack.  The RELAY jack connects the set to other equipment for the reception and retransmission of signals from a remote point."  Nowhere else in the TM is any of this equipment identified or is this operation described.  There is no auxiliary "relay" equipment listed or described in "Chapter 4, Auxiliary Equipment," nor are any related TMs or FMs listed for any such equipment.  The SCR-300 TM makes no mention at all of this feature aside from the call-out in the figure and the single sentence paragraph about it.

 

So...  Does anyone know what this mystery "automatic relay equipment" is (was)?  I have checked TM 11-308 RM-29 to see if it was compatible with the SCR-300, and the SCR-300 is not listed in the RM-29 TM.  However, there is a rather vague statement in the RM-29 TM that in addition to the listed radio sets, it will also work with "similarly designed sets." (Section I, para. 3)  Even if this were the case, the RM-29 uses the radio set's normal microphone and receiver / headset plugs.  It doesn't require a third, "automatic relay equipment." jack.

Anyone have any thoughts or answers?

TIA!