The 54 valve cover was the first year for a four bolt cover. The previous 53 and earier cover, had vents along the center of the cover and used a non vented valve cover oil fill cap. Thoe vents allowed air to flow so the road draft tube could work (air through the valve cover, through the pushrod holes in the head, through the drain back holes in the lifter galley to the block). For 54, its the same except the new design and four screw arrangement. I gues they figured the water draining into the motor from the four side slits was just not a good idea, let alone all the dust and jusk from the air drawn through the motor at speed and so the 55 cover did not have the slits and instead of the non vented fill cap got the vented fill cap that was used from 55 through the 63 model year (really 62 but there was one finl 235 application in 63, limited to a four wheel drive).
