Identifying Postcards by Hugo Brehme
(Germany 1882 - 1954 Mexico)
Hugo Brehme, page two:  Several varieties of three and four-line name blocks can be found printed in blue or black ink on the left side of the postcard backs, as shown below. They reflect a post office box number and the relocation of Brehme's studio from Avenida Cinco de Mayo to Avenida Madero in Mexico City.

Another marking is found printed in purple in the upper right corner, where it was subject to being partially covered by a postage stamp

Of the thousands of Mexican postcards in my general postcard collection, only a handful are marked by the words "La Foto / Madero 42" printed in purple within a rectangular border. So far, each appearance has been on a postcard made by Brehme. One can reasonably assume, therefore, that any Mexican postcard with the following mark most likely has an image photographed by Hugo Brehme.