Who got the other 3 rings?
When Durant got his Flash-Forward, he tells the "pawn shop" owner about his railroad ring, "there are only 4 others like it in the world." Then Collis gives Cullen one and says the same thing about only 4 others like it. So assuming the ones Durant and Cullen have are not the same one, then that leaves 3 others.
One could assume Collis kept one for himself. But that still leaves two others out there. I can't imagine Durant giving one to Mickey (never respected him) or to Psalms (in 2016 he would, in 1869 it's not hard to fathom that he wouldn't).
So who do you think has the other ones?