Original base for 80s Eames chair?  

Yes the base is a reproduction.
Usually, there are rubber shock mounts between the base and the fiberglass. So those may be missing, as the base appears bolted directly to the fiberglass. What's happening in the seat on the other side?
The S stands for Summit Plastics, Kalamazoo MI. So a 1960's or later shell.
This board prohibits evaluations.
Check SOLD side shells on ebay to get a feel for evaluation.

I thought it was from the 1980s. There is a 81 in the white label. Would a 1980s chair still have shock mounts?

I didn't see the 81 the first time around… yes it is probably 1981.
Side chairs from the 80s still had regular shock mounts. The mounts became thin wafers of rubber on some upholstered shells after that, but I haven't seen too many with NO shock mounts. At least not from 1981. The repro base does imply that it was changed up, so my guess is that it did lose its regular sized mounts. Do those bolts go through the chair seat? Looks like they would have to.