There is are basically two versions of the St Petersburg paradox.
They differ only on a minor point, namely in the amount that is paid. In one version, $n$ heads and then tails gives a payout of $2^{n+1}$ (minus the price that was paid to play the game). In another version, $n$ heads and then tails gives a payout of $2^n$.
I'd like to know which version was originally used. I think the letters wherein the paradox was introduced are present in the second version of Ars conjectandi, 1713. However, I haven't been able to find the paradox or those letters in the versions of Ars conjectandi I found.