So, one has what appears to be more vivid colors, the other has a lower dot pitch. Sort of a 1/2 of one, 1/2 of the other situation.

From some cursory research, it seems that the 520 stuck with the passive Sharp panel the entire time, and the 540 stuck with the active Hosiden panel (I'm guessing Hosiden was never a big player in the LCD market, never even heard of them before today). Don't have any data on the 520c. If that one seems to stick with the same panel, it will be odd that the 540c was the only one that had more than one type of panel in its lifetime. Wonder if there was a lottery situation, some got one, some got the other, or it was a wholesale switchover at one point.