Another guitar question...

In the 1973 BBC concert when Bernie and Glenn play acoustic guitars, you can see a separate microphone to pick up the sound. There's no sign of any cable attached to the guitars so these seem to be acosutic only.

In the 1977 concert (the one with the documentary), for "Lyin Eyes", Glenn is playing an acoustic guitar, there's still a separate microphone but this guitar also has a cable. The same seems to be true for the one that Joe is playing. I'm assuming this means they are electric-acoustic guitars. What make are they? Why would they still have the separate microphone?

Any idea when they made the switch?