With the Omniport interface, and the GPI board, and a test peripheral working that meant that the entire design was done. This photo shows the full breadboarded system with annotations:

The next task was to turn everything - except the GPI and test peripheral - into a single board computer. This was a bit of an exercise, as there are 200+ components and 1500+ connections, which resulted in a six layer board. With this level of complexity I wasn't totally surprised that there were a few issues on the board, but none were catastrophic, and there were reasonable workarounds for all.

The board, populated and working, looks like this:

and it works as expected and runs MCM/APL. From my viewpoint, this was fabulous progress.