In my opinion:
This is not a C-21 Pilot assembly. The frames are too short on this unit to be from a C-21. Also the side plate that sits on the journal is curved differently and does not drop down as much as a C-21
Also, does not belong to Uintah 50/51 and does not belong to CB&Q 537. Neither one of these locomotives are close to the same.
SN 34 I cant find a picture with enough detail to see if this is a possibility.
375 Has a lot of similar qualities but it always has the plate for the flanger on the sides. It is too difficult to see any detail that would confirm this. I am also doubtful it came from 375.
Also, the more I look at this, I think the frame in the "assembled" pic is upside down. I believe the flat bar on top is actually the binder and should be on bottom.
Casey Akin