911E Pistons in a 911S

I purchased a 1971 911S that recently had the engine rebuilt. The seller supplied me with all of the receipts and in looking at the rebuild receipt I see that a set of new pistons were installed with the notation "911E lower compression". Why would 911E pistons be used instead of the S pistons. Any help would be appreciated.

bjd71-

Didn't you post this same question a few days ago? Look down a few posts on the board and you will see that pwd72S answered your first post. He guessed it was to lower compression to allow for today's gas formulations.

TT

Thanks ttweed. I'm not sure how this got posted twice.