Just as the earlier posts have stated, the switch inside the microphone completes the audio circuit to the speaker when in receive.
during transmit the switch opens the receive circuit and closes the transmit circuit.
Sure you can ground pin 4 (rx) and have receive audio without a microphone plugged in BUT the drawback is that when you transmit the receive circuit is still closed.
This results in wide open talkback or one BIG squeal.
most older radios relied on the key switch in the mic to do the tx/rx switching, in the newer radios like your TR & galaxy this isnt the case and it is why they work when your 76 and 148 don't.
also in the older radios when people wanted talkback to monitor toys or echo they would install a resistor between the speaker ground and chassis ground.
The more resistance, the less talkback.
if you short mic pin 4/speaker ground to ground... and transmit you will have wide open talkback.
try it for yourself, you wont hurt anything... except maybe your ear drums
there are a couple of ways that you could bypass this so that your rack gear would work with the 148 and 76.
the easiest solution that comes to mind would be to use a miniature relay, NC on the wire going to pin 4. ground the coil of the relay to the chassis ground and feed the other side of the coil with +8v TX which can be found in several places in the cb.
this would let the 76 & 148 work with your rack gear and still work with their microphones