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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