Did all 81's come with a fiberglass rear spring? Mine has Gymcahna and has a 9 leaf steel spring. Is this correct?

Nope. 1981 Corvettes with automatic transmissions had the fiberglass leaf spring in it.
And the remaining 5,000 some odd vettes with the 4 speed manual transmission had a 9 leaf steel spring. Back then they felt that the fiberglass unit would not hold up to the abuse of a 4 speed.
On my own '81, I pulled the original 9 leaf out. (It was tired) and the replacement spring from Eaton Spring was a 7 leaf unit. They just made the leaves a little thicker to accomodate for the 2 less leaves. And I don't have a clue of what pound rating it is. I.E. 330lb or 360lb, etc. I rides nice. Slightly stiff but not bone jarring.
Hope this helps you out.
Regards, Sarge