I reinstalled the 540 panel in the 520, and in Control Panel > Monitor > Options it actually says "640x480, 64 grays" so that footnote on the specifications page is correct. Apparently the LCD in the 540 is a true 6-bit panel.
What does it say under Control Panel > Monitor > Options for the 540c?
What does it say under Control Panel > Monitor > Options for the 540c?