There are two kinds of H74 heads

yes those are H74 air-passage heads
* For air-induction. H74 was used on JAPAN B210 Van 1975-1976 with A12 engine

Induction was used in Japan. Also used in USA and was more common than smog pump (air-injection) which was only used in USA if i remember correctly. Both do the same thing but the pump steals some hp to compress the pump

H74 head came either with or without air passages. With air passages they were usually machined for an open chamber instead of having heart-shaped chambers

Performance same as H72 with same type of chamber. In other words, open-chamber H72 is similar to open-chamber H74. Heart-shaped chambers burn better than open but create more NOx. Open chambers have less NOx pollution but don't burn as efficiently so they added air induction to burn the leftover CO in the exhaust

Later on in 1977 Datsun started switching to high-swirl heads which have low NOx and low CO without need of air induction. By 1982 all A-series heads in USA, JAP or USA were this better type (either using off-set pistons or shaped ports to induce extra swirl)