Sheet music became commonplace in homes starting about 1840. Postcards came along some years later, but when the songwriters put their pens to a score postcards and music were a perfect match.
It’s a hoax! We hear about hoaxes every day, and in this silly season of “hate-politics” the public learns of them through social media. But, not long ago, the hoax was not much more than sport. The Cambridge hoax was talked about in newspapers and on postcards.