This article is from the rec.food.drink.beer FAQ, by John Lock with numerous contributions by others.
There several versions:
The first is that it is the number of words on the label which a
Rolling Rock employee wrote down before sending it to the
artist/printer and it stuck. This is the most popular one.
The second is that "33" is the year prohibition was lifted.
A third, more colorful one, is that the brewery was started with
money won at the track betting on #33 "Old Latrobe", hence the 33 and
horse.
 
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