The plaque above reads:

The World's First Frozen Margarita

...was drawn, using a fabulous new invention...

Mariano's Frozen Margarita Machine

The winning combination of heritage and technology took
two parts of the past and one of the present. The first year,
Mariano's - Old Town saw 36,000 gallons of Margaritas served.

 

VIVA Mexico!, VIVA La Margarita, VIVA Mariano's