Why do old-time beer cans/jugs/bottles have "XXX" printed on them?

I think it was a means to indicate the alcohol content of the beverage, but dun quote me.

I agree with @Symbeline. X was the lowest alcohol content and XXX the highest.

http://www.kibo.com/rawdata/2002/2002-08-06%20Kibo.txt

“XXX” on a jug means booze, “XXX” on a sack means flour,
and “XXX” on a videotape means it costs three times as much as
a movie with an actual plot.”

Guess we got a winner ^_^

EDIT: Guess it’s 404ing, oh well, you get the picture

@Arp Your link did not work.

It’s xtra xtra xtra more likely to give you a nasty hangover.

Most people were illiterate back then and that is simply the signature of the Brewer…. Chauncy Q. Wallingford.

Early brewers designated the strength of their beer with X, XX, or XXX to denote increasingly higher proof or alcohol content.
Source: http://www2.potsdam.edu/hansondj/Controversies/1091124904.html

I think because its mainly for older people and xxx is for adults on like videos and stuff.