Star Trek
Holo-Hohenzollern

13
Thanks to a fortuitous discovery by Captain W. T. Riker of the USS Enterprise, Federation scientists of the Daystrom Institute have been able to determine the Point of Divergence of the fabled Mirror Universe with the highest possible confidence.
The artifact in question is this silver gelatin print of a botched court photograph of Emperor Frederick III of Germany (1831-1919), dated 15 June 1888, taken to commemorate the new German Emperor's reconvalescence after the successful treatment of severe laryngeal cancer.
As luck would have it, silver gelatin possesses unique physical properties that are susceptible to imprinting by short-phase tetryonic particles as generated by an isoversal rupture. Imagine space-time as a planet, and the universe as a Pangaea-like continent; when an isoverse splits off the prime universe (like a continental plate), the dissonance from the phase shift of subspace creates a tetryonic event (like an earthquake in our planetary example). Total space-time volume remains the same, only its configuration changes.
We therefore know that the Mirror Universe's emergence happened on this date, in Frederick's vicinity; according to apocryphal evidence, it was during this photography session that he realized what might have happened in case of his death and early succession by his handicapped son, Wilhelm II. We therefore conclude that in the Mirror Universe, Frederick III died on or around 15 June 1888, and extrapolating from the record of Wilhelm II's short (1919-1924) reign in the prime universe, must assume that his idiocy led to the unfortunate course of history that led to the creation of the Terran Empire.
Using advanced A.I. technology, we have been able to use this silver gelatin print to create an interactive holodeck simulation of this exact moment in time, although limited to the prime timeline, where Frederick III survived. Perhaps he himself can shed light on this moment's significance...