Miller Eames Aluminum side chair in Saran?  

Leave it alone.
Keep it.
Enjoy it.

Rarity
Any sense of how rare this chair is?

Rare means scarce
There are very few original examples left, so by definition they are rare.
I do not know the exact number. But if it's 5 or 20 or 100, it does not really matter, they are still relatively rare. That is why I suggested not trying to repair or alter it, but keeping it and enjoying it.

More info.
Nice find! Here is a post that might provide you with some further insight into your queries.
http://www.designaddict.com/design_addict/forums/index.cfm/fuseaction/th...

Update
Herman Miller is suggesting that it is a 70s chair, maybe in Hopsack. I'm still pretty sure this is a 1958 early issue in Saran, for the following reasons:
1) the fabric is not part of a cushion, but is stretched in layers across the frame, overwrapped like the example on the Eames House webpage ( http://www.eamesoffice.com/vintage/worksheet_detail.php?id=70). Note the fold in the middle of both chairs.
2) I believe that the "patent pending" stamp indicates early issue.
3) The antler is bolted inside the frame (on the bottom), as in the Eames House example.
Am I crazy?

No you are not crazy
You are correct. That is an early example.

Wasnt the Saran scrapped...
Wasnt the Saran scrapped because it stretched/didnt wear well? Perhaps the chair has been reupholstered?

Interesting Question
I've wondered myself whether the chair was reupholstered. If so, it appears to have been reupholstered in something that look very, very much like Saran.
Compare the picture above with:
http://www.eamesoffice.com/vintage/worksheet_detail.php?id=70
The color is different, but Saran came in two colors: gray and blue. Mine is bluish.
So how can I find out?