Since the design was unique, I thought it might be patented. I started my search on the United States Patent and Trademark Office website. Using the index to classification, mailbox design is class D99, subclass 29. I started in the 1950s then went forward/backward each decade.
Crazy, I know, but I ended up pulling EVERY patented mailbox design from 1893 to the current decade! So, I now know that in 1893, W.E. Hartmann patented a Combined Letter and Bread Box (No. 509,532)!
Most of the patents over the last 110 years focused on some novel mailbox feature, not a unique design. I also learned that the term of a mailbox design patent is 14 years.
Of particular interest, in the mid-1960s Sears, Roebuck and Company had a number of unique mailbox design patents assigned to them. The image on the left is an example. In some patent filings, they would site references. For example: Housewares Review, September 1962, p. 89, upper right suburban mailbox item.
The mid-century modern mailbox design that is my inspiration was never patented! But a question still remained: in what decade was the design first introduced?
I did find an image of the mailbox in a 1983 patent for a Mailbox Protector, image on the right. While I have fond memories of the 1980s, I was well past mid-century. But, some of the patent references sited American Home Magazine (July 1954) and Better Homes and Gardens (June 1968), while others sited mid-1970s Sears or Montgomery Wards catalogs.
So, this is where I took my search for the earliest known image of the mid-century modern mailbox design that inspired me!
mid-century modern mailbox 
I think it is such a cool idea that you are embarking on! I also applaud your tenacity! You are so lucky that you can use mailboxes where you are … in Canada, we are soon to lose home delivery altogether, and will have to pick up our mail at central group boxes … more efficient and less costly, and very much hated by the public … You should feel very grateful …