Rare?  

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Not a common color but should be more interesting to
learn a bit about what you have.
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Yes.
It's a pretty rare color. I've had a great many armshells over the years but only one bright kelly green.

Yes
It is one of the rarer contract colors. Even better if it has the narrow shock mount pattern (if it is a side shell).

Shock mounts
Im afraid it has the wide shock mounts. I'm pretty sure the kelly green came from the school desk option. I have just the shell and rather not add a stackable base to it. I saw a guy on ebay that's been making wide mount eiffel tower bases. I hate saying i'm going to add a repro base to it but I think that'll be a good option.
Any suggestions?

Up to you
If you want to use the shell, there are after-market base options available -- stacking, rocker, dowel, and Eiffel tower as you noted. In my opinion, only the wide-mount rocker base looks half decent.
Or you can just keep the shell for posterity... in which case, you might want to get an original stacking or desk base for it.