You're looking at it a bit backwards. Nobody knew what was coming down the line from Apple back in the day . . . or today for that matter, but back then there wasn't a set of specific dates set each year around which to plan purchasing decisions. There were real needs to be met that the Mac failed miserably at providing on the hardware side. It was revolutionary in its Graphical User Interface and WYSIWYG relationship with the LaserWriter and Linotronic, but as a hardware platform it was very much a lightweight until the SE/Mac_II release.
The DTP revolution put production people in dire need of real monitors, RAM and SCSI that the 128k-512ke were unable to provide and they needed them long before the Plus was released. Pagemaker was developed on a 128k and ran in 512k, dunno what the LaserWriters for proofing cost, but Scanners cost in excess of $5k and time spent in the workstation seat prepping for Lino Output was the most precious commodity of all. Vertical market systems often included these kinds of upgrades at time of purchase. They were not necessarily add-ons bought by individuals to keep aging Macs current in the early days. That was a later development, user installable upgrades were a much later development.
I spent almost $10k on the MacSignmaker System. This was an SE/20/1M with the Radius16 Accelerator, a Video Digitizer, a B&W security camera with a decent lens, a cheap tripod and a closeup filter pack. The heart of the system was a stepper driver card wedged between the Plotter Cable Pins and Plotter Cable of a Gerber SignMakerIII, or in my case, the second generation Sprint. The added 8MHz was wonderful, but the Math CoPro on the Radius16 was a dire necessity, something unavailable on any stock Mac but the Mac II, released along with the SE. I figured the system would pay for itself within the first year simply by computerizing the pattern making process. That's how I sold my partner on it. The Logo digitizing for Vinyl Cutting was gravy on a meat & potatoes process.
You used to be able to run a TTL Monitor off the Mac's Video Interface, but I never saw anything that ran off a BNC connector hooked up to the Power Connector. Buzz the connections, check to see if the video signal is what's running from that double level power connector or if someone hijacked one of the 12V pins for something else. I can't see any indications from the pics, have they shoe-horned a signal to the missing pin on the Mac Cable and run that to the BNC signal Wire?
I'm still thinking ThinNet if there's no clear indication of a Video Card.
A diagram of all the patch wires heading to which MoBo positions from where on the cards might explain quite a bit.