Name change:  Who knows the origin of this?

This goes back to a time when common women (not royalty) didn't have identities in their own rights and upon marriage just became something akin to a child in their husband's family. 

That's old school 50's misogyny crap.  

It's quite an old custom, which only began to change in the 1970s and 80s. It dates back at least to the early 1800s, maybe even further.

It probably has roots all the way back into royal and aristocratic titles in the Middle Ages. Even today, this custom persists among British royalty and aristocrats. For example, the official title of Kate Middleton is not Princess Catherine; she's officially, "The Princess William," and Meghan Markle is "The Princess Henry"  instead of "Princess Meghan." I suspect that common people simply began copying the custom at some point to make themselves seem more cultured and genteel.

I am SOCIALLY Mrs. John Smith.  Legally I am Sandra Smith, no "title."  I've had to explain that I, Sandra Smith, AM Mrs. John Smith and, therefore, because he signed the appropriate legal forms, I can speak on his behalf.  Maybe it's easier to just say, "I'm Mrs. John Smith."  When women had NO identity other than "Mrs." someone this was the common practice.

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What do I do when someone calls my firm and says, "I'm Mrs. John Smith"?  I say, "And may I have your first name, please," and life goes on.

I guess her husband dumps so much on her she feels like a John.

Chauvinistic tradition, it is on decline. There was a time women in the U.S. could not vote, could not own property or get credit in their own name. It has been eroded over time, but older women still do that.