Okay, I read that the 2-letter prefix on the TTL logic chips just denotes the manufacturer, so I removed A1 from the PO board, added a socket, and pulled the GD74LS374 from the parts board and installed it in the PO board. The good news is that there are no more corrupted graphics. The bad news is that there are still some major problems. This is the result (for some reason I can take focused pictures of the bottom monitor, but I can't get a focused picture of the top monitor to save my life ... in reality both monitors are in good focus):
So on the bottom monitor there are screwy colors, and the "Nintendo" text on the ring post is missing. The top monitor is in black & white, and there are a lot of missing graphics. Here are some more missing graphics on the top monitor from other screens:
Out of the three PO boards I have (1 converted to SPO), 1 of them had a color issue that I assumed was bad color proms, so I guess this particular board ended up with the bad color proms after I did all that swapping around to end up with 1 working PO, 1 working SPO, and 1 garbled graphics PO (which is the one that's the subject of this thread).
However, bad color proms wouldn't cause missing graphics would they? I'm guessing that not only does this board have the bad color proms, but also whatever components went bad to cause the issue that is the subject of this thread.
Should I continue on with ...
Replace 5C,6C,7C
Replace 6A, 7A
Replace 5A 5B 6B 7B
... or do the results from swapping A1 and the possible color proms issue change things?