The idea behind the RMS is pretty much the same as for Gus. Before January 1st 1901 they were just three old fruitcakes in an office at the back of the British Museum. After the green comet appears, nothing will ever be the same again. (A bit like Vince Cable now that the economy actually has collapsed - see, bet you wish you'd listened sooner.)
The girl in the pic is not Estelle by the way. She's a whole other bit of skirt (in this case, armoured skirt) for Jack to hang around with. See, I learned my trade from Stan Lee, and the Gwen/Mary Jane dynamic made a big impression on me ;-)
Some people have categorized Mirabilis as steampunk, incidentally, but I never thought of it that way myself. Steampunk as a genre postulates a world in which Victorian technology takes off in a direction that allows airships, robots, and all that - er, not jazz. (Ragtime?) Anyway, we do have steam-powered robots and so on in Mirabilis, but we also have Martians, faeries, gods, goblins, ghosts, witches and outright Dunsanian whimsy. Like I was just saying: before New Year's Day 1901, this was our world. The real world. Now it's the year that history forgot. "Everything is going to change." Keep that in mind.