[Agp.cooper] saw a vintage 4Kx4 bit RAM chip and decided that it needed a CPU design to match. The TTL design fits on two boards and has a functional front panel.
This custom CPU project has a few interesting bits worth noting. First, it is small enough that you can wrap your head around it pretty easily. And [Agp.cooper] gives a good account of the instructions set architecture choices he considered and why he settled on the final design.
Another interesting twist is the testing. Each of the boards can mate with an Arduino which will exercise and test its functions. This allows each part of the design to test out before trying to bring up the whole CPU.
The PCBs are from EasyEDA, a service we looked at awhile back. The boards for this offering ran about $70 total, but [Agp.cooper] mentions the cost is reasonable for a one-off project, but the two-week turnaround begs for getting it right the first time lest your project sit dormant on the bench during a respin.
This is the same designer, by the way, that did the Wierd CPU, and he compares it with his 4-bit creation some in the build log.
Thanks to [starhawk] for the tip.