I went ahead and temporarily removed the mods and bridged the cut trace, but the system stops with "Fatal system error-> 0911". I also socketed the 4464 sound RAM and replaced with some known good chips, but I still get the System Bad: 0C000001 error and screech during diagnostic (with the mods reinstalled of course).
As for the revision, the board is marked 607-0173-B (B stamped in ink) on the right side of the board near the slots, and 820-0167-B under the solder mask on the front of the board. Thanks much for your assistance in confirming this is unusual - I may wind up looki