A couple of notes about the PLC:
(Note: these are semi-informed speculation having used PLC's in this era).
The part which daved pulled off which had the EAR and MIC jacks on it is likely just a programming interface. Because of the cost of the programming interface, many plc's had a separate programmer which you would take from device to device. The D25 connector on the programmer is likely for a printer to print the program out. And the EAR and MIC jacks - well that's most likely to save the program to a cassette tape.
RY I'm about 99% sure is short for 'Relay'. Oh wait, I just got to that part of the video where Dave figured it out. So I guess that's not much of a revelation.
One note about the architecture: The PLC itself has a processor with the eeprom with the actual 6800 code in it. Through some sort of proprietary interface between the programmer (which has a separate CPU which knows how to do the display and all the programming interface stuff) and the internal CPU, you would load the logic program into the RAM chips on the CPU board. That supercap most likely holds enough power to keep the program in the SRAM during power outage. Once the programmer loaded the code into the processor board inside the plc it could be removed and the PLC processor would run the entire show without the programmer needing to be attached.
One additional note:
On the programmer near the DB25, you'll see a couple of IC's with 75188 and 75189 on them. Those are IC's which do RS-232 to TTL and vice versa (one for each direction).
I recognized those chips immediately as I swear those IC's were the bane of my existance in the late 80's.... There was the Motorola MC1488 and MC1489 which I think were first, then other manufacturers came up with their own pin compatible versions which were all numbered differently, except they all seemed to end with '88' or '89'. SN55189, SN75189, etc. The driver needed both +12V and -12V rails, and the receiver only needed +5V.
They also were not very robust. On longer serial links (or sometimes even on a short one), those parts would die. Or for a short, or because they felt like it was time to retire. Who knows. (I think the phase of the moon had something to do with it too).
As a result, it wasn't all that uncommon to see just those two parts socketed on a completely socketed board. Those were two chips you always kept on hand - and if one which wasn't socketed failed, you sure put a socket in place instead of just replacing the IC. This board obviously has been spared.