Gotta admit, am confused again….The original style thermostat gasket IS available at ME:
http://www.marineengine.com/parts/johns … +Crankcase

There is room for plenty of confusion here, and I’m not sure just what Joel is describing, so will review the usage on the three gaskets.
The original gasket: 318917 is used on 74-76 engines that never had the cylinder head update. The key here is to be able to identify these cylinder heads when you see them…Like Dan says, the early heads/covers are not "scalloped out" by the spark plugs. Needless to say, these pieces must be used as a set. In other words, you can not mix and match old heads/covers/gaskets. It surely is possible that some folks have had the old heads/covers laying around after doing all the updates on these engines years ago, so it is possible that older style heads/covers/gaskets got installed on newer engines. an example of this might be a newer engine having a head replaced due to stripped plug threads or broken water cover studs in the head. Although this is surely an error, it could be done, and I’m sure it has been done. I can see repair shops using older pieces laying around the shop instead of ordering new/correct parts.
The next generation of cylinder heads/covers used gasket:321387 (yes, I do realize this gasket supercedes to the newer gasket: 329920, more on this in a bit) This generation of head/cover/gasket was used until the mid 80s. Changes were made when the relief spring was added on top of the thermostat, but this did not change the gasket design/part number. This gasket was also used on the original 74-76 engines with the updated cylinder head/water cover.
So, what changes were made that required the new gasket: 329920? The port/bottom side of the water cover was prone to external leakage, especially during servicing. The cover was subject to warpage/distortion during servicing, and there was no retaining bolt holding this area of the water cover securely to the head.
So, in the mid 80s, another change was made to the head/cover/gasket that added another bolt in the port/bottom area of the cover to help minimize external water leakage in this area. Needless to say, these pieces (head/cover/gasket) were unique as well and could not be mixed/matched with different style pieces. This generation of pieces requires the new gasket: 329920.
I guess the next question would be: why does the second generation gasket (321387) legitimately supercede to the newest style: (329920)?
The only difference between these two gaskets is the added gasket material and hole for the extra retaining bolt. So, yes, when you use the newest gasket on the second generation heads/covers, there is a piece of the gasket hanging out of the bottom/port side of the cover with the hole in it for the extra bolt. The new gasket will work fine in these situations, because there is gasket material sealing the surfaces of the head and cover in this area. Yes, it looks wrong/screwed up, but you can simply trim the little bit of gasket off after reassembling the head and cover.
Again, you can not "reverse supercede" parts, just like always. Dealers love to cross out original part numbers on NOS OEM parts, and write in the newest part number. This is a mistake and should never be done. Let’s just say the dealer had a bunch of the second generation gaskets: 321387. He noticed the part number change and wrote part number: 329920 on this envelope with five of the second generation gaskets he already had in stock. Now, let’s say someone with a late 1985 engine came in for a new thermostat. Mistakenly using a gasket from his envelope of "dealer superceded" gaskets would cause a possible leaky water cover because there was no gasket between the head/cover in the bottom/port area.
Needless to say, the same thing could have happened to a newer engine that I described earlier with second generation engines. Perhaps someone brought in a 1985 engine that needed a new cylinder head. Instead of ordering the "correct" head, the dealer decides to use/save money by using a used head off a 1981 engine that was blown up. In this case, future repairs might get confusing because water covers/gaskets are not going to fit properly.
Needless to say, there were other changes made to these heads in later years, but these pieces are easy to identify/not so easy to mix up.
Joel, once again, not sure if I am answering your question, hope so….The key here is to be able to identify these parts by looking at them, to avoid confusion.

We used to use the middle gasket in place of the left gasket all the time. Like you, we were confused at first; but there was really no other way out. If the surfaces are true and you use a thin coating of perfect seal, there really shouldn’t be a problem. We never had one. Like Fleetwin says, if you just trim off that extra hole on the bottom right, it is then really the same gasket. If it leaks, it’s because someone wasn’t careful cleaning it up or the motor over heated and the parts warped. (I mean, not to say that 9.9s and 15s over heat much, but…..Lol. In salt water, what one has NOT overheated?) If you keep using the old heads, you invite a leak at the bottom from the old design, without that extra bolt. No doubt that’s why they changed that part of the head design….

As for the Sierra 18-2905-9, I could have sworn that the dash nine just meant it was the same gasket sold in a bulk quantity, like five or ten of them, or something. Correct me if I am wrong about that….

Joel
I’m not seeing/understanding what you are trying to point out…
Just trim off the excess gasket intended for the latest style head. I guess if you left the excess gasket in place it could snag on something and perhaps get pulled out.
I have experienced leakage in this area also, even with the original gasket. It is tough to get these mating surfaces true, so I have been using the OMC typeM cement that will better handle these imperfections….