The modification to the Apple IIGS sound circuit to reduce the unwanted
background noises from the port was on the Revision "C" logic boards.
Unfortunately, due to some parts-availability problems, there were only
34,000 Rev. "C" logic boards produced.
Production has subsequently reverted back to Rev. "B" boards. However,
service stock logic boards are modified to Rev. "C" when repaired by the
Apple service repair centers.