See here for some info on the movement historyL
http://chronomaddox.com/moonwatch_cal_background.html
And yes, there are a small number of 3590' swith 18 jewels, but I believe they are very few. I think that all 3570s have 18 jewels, but I'm not 100% certain as I'm sure that somewhere out there a parts bin special just happened to be assembled by Omega with an old movement.
And yes, the 18th jewel was added before the 8xx to 18xx changeover.
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