American Standard 4066 Toilet Part Guide

There were 2 versions of the American Standard 4066

The First Was A 3.5 Gallon Per Flush Toilet

The Second Was A 1.6 Gallon Per Flush Toilet

You may be able to see a stamp in the tank telling the 1.6 GPF
The 3.5 GPF was an older toilet with a large tank of water