1869 - the Japanese Emperor was moved to Tokyo in March 1869 and Tokyo became the Japanese capital from that date... but I cannot narrow it down to the 12th.
One other thought - Mendeleev presented his Periodic Table to the Russion chemists on 6 March 1869; could it have been accepted elsewhere on 12 March?
Don't know if you've already thought of and discounted these, but supplied in a spirit of helpfulness.
Much appreciated Big Mac, of course you are welcome to join in-4 brains are better than one. You might like to know that the quiz is a new one for the New Year, and the emphasis is therefore on the answer being a "first". I tend to find that more people get involved if they think answers are general knowledge (which a lot of the rest of the quiz is ) than if a quiz has a specific subject that they know nothing about.