Tom,
You are 'absolutely correct. I think that all 3 of UP's steamers have many parts off of other engines, and sometimes not even from the same class, so long as they were interchangeable. I was just thinking about the NG survivors, as we have several of each class but not so many as to make a parts catalog an impossible task.
On the other hand, during our cosmetic overhaul of Red River and Gulf #106, we have to date, not found a single part from any other RR&G or Crowell locomotive, even though there were 3 other Baldwin 10-32-D 4-6-0's and 2 class 10-30-D 4-6-0's on the combined locomotive roster of the two companies and they were all serviced and repaired at the Long Leaf shop.