Leon, I believe the postcards Meredith was so proud of were not the ones with his building and mug on them, but some "Art, Birthday and Season postcards" which he referenced as premiums in this early 1916 subscription letter.
I have never found any, although I do not look hard or often; however, there are often these kinds of poor-man's Norman Rockwell postcards on ebay that were put out by Successful Farming during that time --some may have even been used for the magazine covers. (these are on ebay now):
BTW, Meredith was quite a guy, and his family's corporation still exists and still publishes Successful Farming, along with dozens of other mags and with ownership of several TV stations. He helped create the 4-H Club and was US Secretary of Agriculture during Woodrow Wilson's last year in office.