There are four different cylinder head castings [that I know of] for the E-engines. The casting # is on the exhaust side of the head, just to the left of cyl. #1 exhaust port, [at the base of that one "long" head bolt]. The earliest one is the 31M casting. For high performance applications, this is the least desirable casting, as the intake ports are heavily shrouded in the bowl area, to promote a "swirled" intake charge. These were on all E15S and E16S engines up to about '84. This is when the 33M heads came out. They are reputed to be worth 5 more horsepower than the 31M heads. The reason is that the 33M head has straight, clear intake ports, without the "swirl" shrouding in them. This head was used on E16S's and E16i's until the end of US market E16 production in '88. The third head was the 15M head, which is the beloved E15ET Pulsar Turbo head [or Cherry Turbo-depending on what country your in]. This head, has the smaller 17.9cc combustion chambers, larger valves, and is reputed to be a thicker casting, to deal with the extreme heat associated with turbo engines. An additional benefit of this is that this allows them to be ported a bit more than the others. The last head that I know of is known in the Nissan Motorsport catalog as the "Fuel Injection" [or FI] head. These did not come on any US market Nissans. It also wears the 15M casting number, but that number is not visible on the outside of the head like the others. It can only be seen under the valve cover. This head is essentially the same as the Turbo head in every way, except that it is a "thinwall" casting, for normally aspirated use. My understanding is, that this is a European O.E. head, where emmisions are not as stringent as the US versions. These heads were sold through Nissan Motorsport [I bought three of them], but are now NLA [no longer available]. About five years ago,I actually bought the last "FI" head, and was told by Nissan Motorsport, that they now are selling actual Turbo heads. And I'll tell you, that the price went way up too! I'm not sure why the latest Motorsport catalog still calls them the "FI" head. If anyone knows of any other E-series cylinder heads I haven't mentioned, please tell us. Especially from other markets around the world. I'd love to know about it!