I am not sure exactly what the number in the DEV test means, but likely relating to the address the error occurred.
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That makes sense. I looked things up and IC73's address space starts at 0x26000. I tried 6 different RAMs consisting of 3 different part numbers and they all failed a 0x2669B4. It's really weird that it always fails at the exact same location. Or more likely it's not weird, I just don't understand what is going on yet.

Maybe that is just the first place in RAM that it stops the test for that IC and reports an error.
So, I took a look at the data lines on IC73. This is DDB15 on pin 19. What the hell is that sawtooth doing there?!? (The scope is grounded directly to pin14 of IC73) This was on all of the data lines of IC72 and 73. It is not on the address lines and it is not on the data lines of IC129/130. (I don't recall for sure, but I think it was on IC54/55 data lines) Any chance you have a board on your bench that you could look at the same waveform?
This is the same pin on IC130. Different time scale, but you can see there is no sawtooth.
These signals run through at least 2 EPROMs and 2 RAMs so I went upstream to the source of the signal, pin 2 of IC59. (Taken at a different time, so the pulses are not expected to be the same)
So the farthest upstream I can go is directly to the 68000.
This is pin 5. Pretty noisy... I need to overlay the 2 signals, but I think this explains the sawtooth coming out of the buffer.
I looked at the power and ground of the 68000 and there was nothing interesting, so I went for broke and swapped the two 68000s. No change, still fails at the exact same address, 0x2669B4.
I ran out of time before I had to leave for work, so I still have a lot of ideas to investigate. Maybe there is a bad decoupling cap somewhere that is bad. I see a 10 uF tantalum on the 68000 VCC. Also what looks like a ceramic in the schematic, but... I can't tell the value.
Stay tuned...