I've been trying to connect a Macintosh Color Display (M1212, 640x480 @ 66.7) to a Cube. I have finally succeeded, and wanted to share in case anyone else ever wants to do something like this and doesn't want to source an adapter or has an adapter which is incorrect for your particular monitor.
While monitor pinouts for DB15/HD15 are common knowledge, I found this particular adaptation to be confusing, and resources on the web extremely scarce. There is one Apple Developer doc that *hints* at a solution. My first dozen attempts at wiring it myself failed, and the first adapter I tried (590-0322, for 17 inch multi-scans I think) didn't work, even after cutting it apart and trying some re-wiring. The proper adapter that I should have gotten to begin with is the 590-1120. Testing for continuity on all pins reveals the solution:
3 13 (HSYNC)
4 4 (ID 0, when adapter is attached)
7 15 (ID 2, when adapter is attached)
10 12 (ID 1, when adapter is attached)
12 11 and 14 (VGA ID and VSYNC)
14 10 and 5
15 connects with DB15's 3
This was the same as 590-0322, except that the GND on 14 went to 10,5, and 11. I was able to make this adapter work with my monitor simply by snipping HD15's 11 and jumpering with 14.
Although Apple's Doc for its powerbook G3 adapter (the doc for the Cube's adapter, different part number, does not mention this) mentions that the computer looks to pin 11 first- if it is attached to VSYNC, it knows the adapter is attached and it looks to the traditional monitor IDs.
What I found to be not-so-obvious, and why I guess my own wiring didn't work at first, are the connections between 14 to 5 and the V and HSYNC connections. I had assumed that a DB15 monitor only takes composite sync so I didn't hook those up- I just combined them into pin 3 (which has worked for others, like in the iMac/Color Classic hack). Something about this didn't work here, perhaps because the link with pin 11 would become composite and not VSYNC, causing the computer to not realize that it should be looking at the old-style ID pins. Perhaps something inside the sizeable monitor cord connector keeps that signal from flowing back up the wire- after all, the only signal going to the a/b is composite (I've dug around in there), so it gets combined in the cord.
I hope this helps somebody avoid having to buy an adapter, or to be able to fix an existing adapter that is for a different monitor. I'm also interested in learning more about how the sync stuff works, if anyone knows.