(coexisting on another dimensional plane)
Upon detection of Cochrane's warp signature, a Vulcan survey vessel altered their course, landing in Bozeman, Montana, the site of Cochrane's launch. Exiting the vessel, one of the Vulcans attempted to greet Cochrane, but was killed when Cochrane pulled out a shotgun, rallying the rest of the townspeople to board the vessel and appropriate any item of value. This action resulted in the early expansion of the Terran Empire, allowing Humans to drastically increase their technological capability and begin a campaign of warfare and conquest.
As of the year 2155, Starfleet officer Maximilian Forrest, captain of the ISS Enterprise, was in possession of the shotgun used by Cochrane. In that year, it fell into the hands of Commander Jonathan Archer, who had executed a mutiny on board Enterprise, taking command for himself. (ENT: "In a Mirror, Darkly")
Background information Edit
Cochrane was played by James Cromwell, who appeared through the use of uncredited archive footage from Star Trek: First Contact. In shots where Cochrane wields the shotgun, the role was played by an uncredited photo double.
The final draft script of ENT: "In a Mirror, Darkly" makes it clear that the mirror universe Cochrane would appear via reused footage from First Contact, but does not make any outright statement that this version of Cochrane was meant to be from the mirror universe, leading the reader to assume that from the actions described.
Zefram Cochrane was likely the first Human to achieve warp flight, much like his counterpart in a parallel universe. Just as in the series' regular opening credit sequence, a clip of Zefram Cochrane's ship, the Phoenix, from Star Trek: First Contact, is seen in the altered opening credit sequence created for "In a Mirror, Darkly".