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davidcyclebackWhat is said above is correct. Many real photo postcards were basically family photos for the family album and were not meant to be sold to the public. For families and home town photography studios and photo developers the standard postcard size photopaper was convenient for such photos-- a nice size. Some of these family photos were mailed to other family members and friends.
So some real photo postcards were meant for mass commercial consumption, while others were family photos or snapshots simply developed on postcard paper. For the latter, the postcard text and design was almost incidental.