Difference between a PRE HE Flathead V12 and a HE V12 ?

[TWINTURBODAVE]

Hi...I have just bought a 1978 PRE HE XJ-S....now, I understand about the later HE engine having the recessed combustion chambers in the top of the piston...for better fuel economy...but..whats the 'Flathead' in reference to?..is this the pistons...or.is this the cylinder head itself?...i guess its the pistons, but would just ike to be sure...thanks