Special days required special art – the upgrade was probably a penny or two. This 1937 example has boilerplate text – give Ma message number six, she’ll like it – but it was special enough that the recipient saved it for years in a scrapbook.

Note that the sentences don’t end with STOP like the old cliché; a period suffices, as does the equal sign. It’s a rather interesting use of punctuation; ending a sentence with the equal sign has the inconclusiveness of a semi-colon, but nevertheless seems to indicate that this particular matter is closed.