No, the flat part of the chamber is the part that provides the squish, which in turn provides the swirl & turbulance of the fuel /air mixture.
This promotes flame propagation & reduces detonation. The upshot is that the more that you can controll detonation, the more you can raise the compression.

I think the 1971-1973 A12 (non-GX) heads came either way: open chamber or closed chamber. Maybe it depend on the year, or maybe the country.

the GX head has oval ports thet are TALLER than standard oval poirt heads. Also the water orifaces under the intake ports are LARGER than the holes in an A15 one.
The valves are indeed the same size as stock A12 ones, but a head shop can fit A14/15 ones no prob.
And as D/MAN said, some do not have a casting number on them, (The two I have seen did not)

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The Nismo head had no casting number and has the inlet port roof protruding into the rocker area.
nismo head raised inlet ports

Round port head 990-459?
What was this off? I have it on an A15 now...

which head is beter out of the a14s the one i am using is a h75 i havent heard much about it but it bolts straight up to the late modle a12 block and is a great improvment

jus a case of browsing again...i have a head with the Markings H85 and 503 what sort of head would this be?? i havent removed the manifolds as yet.

H85 designates: B310 4-dr sedan 1200 & 1300 CT & DX (reference: A-type Cylinder Heads).

This is probably a round-port head, you can tell by looking at the intake manifold -- are the runners round or oval where they enter the head?

Remember, round-port heads are best for low-speed horsepower, in case you prefer not to keep the engine revved up all the time.

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me too ??? well not exactly using at the moment ... deciding on how far to go with it .... i think i've just about read all the stuff in the tech section re:cylinder heads to no avail .... any info would be good ????? mine has the casting numbers H75 308 ..... mr ddgonzal ??
ps : where can i find the valve size used in a L16 head ?