As I understand it, a branch of the Platt family relocated to Eldred, PA (?) and through subsequent incarnations became one of the Union Cutlery Co's, which subsequently was bought by Case, and one of the brother's went to Case... only to leave relatively soon thereafter...
As to the Eldred, PA knives, very few were marked either Platt or Eldred because they were jobbers - they'd accept an order and stamp it with someone else's logo, as Case does. So I also presume the Case knives marked C platts and Sons is but a tribute to one of the major forces behind the early Case story... ? Again, only as I understand some of that history...
PS: See the current edition of Knifeworld (IIRC) for more on the Platt story...