the later reversed cyl heads were indeed belt driven and were also a special cast and cnc machined race head, there are NO similarities to a standard de race prepped head.
the inlet ports are high and huge and drop straight down onto the valves, as in the port is as high as the top of the cyl head.
there is also a series 1 & 2 cyl head, the series 1 uses stainless valves and the series 2 uses titanium valves as well as a few other tweaks.
these heads do not have a nissan part no`, they were commisioned by team nissan europe (outsourced ) for the later p11 btcc cars , which were run by ray mallock in the uk.
they also dont even have the blueprints for these heads any longer as they were lost/ destroyed at some stage.
my engine builder has tried to duplicate the flow rates / porting etc into a de & ve cyl head and cannot as there is not enough meat left in the heads .
thats about all i know about these heads, except that i own one which we have converted back to conventional drive so we can use it in a rwd car.