Fiberglass Eames need some love  

The epoxy is almost surely original. A lot of HM shell chairs look like that.
I think these are on the newer side, because older chairs I have seen are date-stamped in the format OCT 6 57. Later ones have the date encoded, and I'm not sure how the codes work; 861902 may mean February 19, 1986, but don't take that as a fact.
IND GRADE may have been added by the buyer, not the manufacturer.
Show us the stamps? That may help.

Photos attached. Were they making the fiberglass chairs into the 80's?
Also of note, they are stacking bases, but do not interlock side to side.

These particular chairs are 1960 or later. The circled S and raised letters began in around 1960. And yes, the side shell chairs were definitely made well into the 1980s.
I would not wet sand and re-coat the entire chair. The seat side looks shiny and serviceable, and at least you can say the surface is original, even if not much else is going to be.

how do I know if the stacking bases are zinc or aluminum?

Well, your bases are zinc-plated steel and appear to both stack and lock together side-by-side. I remember seeing somewhere aftermarket dowel-leg bases configured to fit wide-spaced shock mounts. Perhaps a little web searching will determine if these are still available?

I bought my wide mount dowel base from Modern Conscience without any issues, but see the post below about them before you buy.

I minimally cleaned the chairs (soap and water) attached all the legs, brought them inside, and ran a cloth across them to dust off. When I looked at the cloth it was covered in loose fiberglass...and my arms were itchy. Short of wet sand/Penetrol what can I do to them to seal in the fibers?