Where does the "I" and "E" go on the top of the air cleaner?

it depends if you want to draw in cool air or warm air

Trivia time - what do the letters stand for?

I = Inverno = Winter
E = Estate = Summer

I = Inverno = Winter
E = Estate = Summer

So the idea is you have to decide if you want hot air (being drawn in from over the exhaust header) or cooler air (drawn in from the center of the engine compartment) to enter the carburetor. The top of the air filter housing has a "block plate" on it and the air filter has to "sources" it can get air from. In the summer, you (typically) want the "cooler" air entering the carb, so you turn the air housing so the block plate "blocks" the inlet coming from the exhaust manifold, and allows cooler air to enter from the engine compartment.

In winter, you want warmer air going into the carb, so you rotate the air filter housing so that the block plate "blocks" the cooler air passage and opens up the path to the hotter air from the exhaust manifold.

Here in so Cal I just leave it in the "I" position at all times (to get the cooler air). However, if I lived in a cold-winter climate I'd rotate it to "E" during the winter.

Hopefully this answers your question.

Peace.
Out.