I'll hazard a guess that nothing was changed.
There were probably several things that should have been changed, but as gerd likes to allude to so often, Fritz apparently pulled the wool over the eyes of the Feds (with help supposedly) and the cars may have entered the country untouched, save for the cosmetic changes that have been removed from every single "Ameritech F1" that I have seen. In fact, I believe gerd even claimed that not every car was actually modified with the strange lights, hideous bumpers, and side seat covers like they were supposed to be. Why take the time to do it if you're just going to be removing it, right??
I assume the reason you are asking is because of the difference in performance figures of the Ameritech F1 that Road&Track tested on two occasions versus the numbers we are all familiar with from the AutoCar test and the Ehra Lessien top speed run. It possible that the car they tested just wasn't performing at its best given the heat or the quality of US fuels. It's really hard to say...
I expected that he would chime in here, that's why I left this one alone for a while. He knows more than he tells us and might even have solid proof of his claims, but I doubt we'll ever see it presented here. Oh well...
>8^)
ER