Printing was a business and booksellers were anxious to sell their stock. Conveying information through colophons served as the printer's primary means of making buyers aware of what he was selling. A basic colophon included the essentials: the place of printing, the printer's name, and the date of publication. But enterprising  booksellers expanded the traditional limitations of the colophon to inform readers about what else was available in addition to the suelta in hand. This was done in one of the following ways:

Some printers enumerated in their colophons the great variety of goods they carried. Most of these were performance related: Comedias Antiguas y Modernas, Saynetes, Entremeses,  Autos sacramentales, Tonadillas, and Jácaras. But they also sold Estampas, Historias, Relaciones, Romances, or just about anything--Papeles muy curiosos.  To entice buyers, some included price per item and discounted price for volume purchase, with or without binding.  

Densely printed, difficult to read, some early colophons included the titles of specific plays that they printed and had in stock. This later evolved into a list following the colophon to fill in the rest of the page or maybe on a separate page. 

Later in the 18th century, when the business capital flowed from printer to bookseller, the list grew in length and reflected what booksellers had on hand regardless of who printed it. These lists are often two pages long, in rare cases longer.