I found these in my gas tank. I think there might still be a few more inside the tank. I'm thinking they are lead because they are non-magnetic. Each has "GS" stamped on the bottom. Maybe some kind of lead additive? I'm at a loss...
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Buick?

By all means put them back in. They will raise your octane level by 5 points and increase your gas mileage to 50+ mpg.

I think they are one of the Snake Oil gimmicks that were very common in the late 70s when fuel prices shot up.

Brian-
Are these the super rare Buick Grand Sport racing tabs? Worth a Million a piece? Man, you should of bought my car when you had the chance...hahaha

Kyle

As the saying goes, I could tell ya, but then I'd have to...

P.S. I still blame Chris

Carbonflo Pellets?

http://www.mgexp.com/phile/1/223560/IMG_4113.JPG

Link didn't work.
Try this one.
http://www.mgexp.com/phorum/read.php?1,2546690,page=2

Ballast? Noise makers?

Cocktail shaker leads?

My car is a hard top, but who knows

In the seventies and eighties they sold tabs that were supposed to restore your engine´s performance while you were driving. It was called Motaloy, so it doesn't match the letters GS that is on your tabs, but otherwise they look nearly the same, to my eyes.
I googled Motaloy and found this: http://www.thesamba.com/vw/forum/album_page.php?pic_id=811172

Maybe your bird developed "Gas Stones" instead of kidney stones