The writing on the back states that it was received by one of my mother's aunts from another of them the day after the latter's marriage (which the former attended).
Given that this was two months after the future George VI married Elizabeth Bowes-Lyon, my guess is that the aunt that married had a stash of them ready to give out before her own (rather more modest) wedding photograph was available.
The note on the back also says that it was "To keep in remembrance of a good time in London" with the man the second aunt would marry four years later.
It's nice to know why it was meaningful, then, but nevertheless a little weird to discover that it actually was.