Further to fitting the D breather on the front spring cap. The first photo shows a C front head and it has both the crescent shaped reliefs top and bottom. The second photo shows a head that came out of the factory quite late. It doesn’t have the relief on the bottom so more restrictive if used with a D breather cap. Not much air gets past the top relief because it is almost sealed off by the rocker bearing. Without the relief on the bottom, it severely limits the ability of air getting through the two drain grooves, so more air would be forced through between the rocker bearing and rocker arm. It would be interesting to see a photo of a known original D front head. Was the bottom relief left out intentionally or was someone asleep at the switch.....