checked, after changing the input signal mode to "PC" at the 500a, the 500a could show the "1920 x 1080 24Hz" message while playing the 24hz blu-ray material. But the 1080p/24 still could not be shown with the 51FD.
B4 changing the mode to PC,
500a Display as follows at the top right-hand corner :
HDMI
1080p
36bit