The first sample cards (Venetian, from ca. 1825?) were made to be circulated at international expositions—such as happened at various locations in Europe in the 19th century. What became The World's Fair in the 20th century—such as Expo '67 in Montreal.

Some of these have been shown by Jargsdorf. Perhaps other authors.

By ca. 1850 Venetian cards were used as marketing devices—and, as you know, were sometimes for general distribution, and sometimes for specific markets.

To find and examine the earliest cards, I would seek the more-fancy "souvenir" editions, that proclaim the range of products, but were not intended to ilicit sales, per se.

It has been my attempt to "date" cards with no explicit dates, by the technical quality and styling of the cards themselves. And whether they have been hand-written or printed. And, of course, through great collections—such as at the Tropen Museum in Amsterdam.

(I have not read the other replies. I'm really here today for another purpose. But I will read them. Please forgive any duplication of ideas.) Jamey