Correct. Service Manuals were printed on the quarter year, corrections included. Same with PARTS MANUALS, upgrades every quarter. Corrected service procedures were also released with SERVICE BULLETINS.
But another poster showed a page from his 54 SHOP MANUAL (don't know if FORD or MERC) that showed the correct sequence.
It may be the ILL is first print and the later 56/ rocker arm adjusting screw ILL inserted on that particular ILL.
BASIC PN 6A506 is the BPN for a valve cover set incl grommets. The grommet is also available separately.
Never say never regarding FOMOCO.
W (BY GOD) V - EASTERN PANHANDLE