Hmm but what impact would that have on the story, I mean sure it would be cool yo learn more about them but in Ragna's back story it said that he's destroyd 4 already and the two we do know of play a big role but the back story would have no impact on the plot.
Take the noble families sub-plot not much has been done with that apart from the odd bits. I don't know one day if Mori finally decides to troll us all and actually go in-depth with the series and mind fuck us even more :8/:, then I think it would happen but who knows? Maybe we'll see more of the Murakomo units in the future.