Jeff,
I am aware that parts have been shuffled around the K-36 and K-37s for years, especially running gear parts, and probably all of the surviving NG locos have many parts from their brothers and sisters as well. It would be an interesting project for someone with the knowledge and lots of spare time to compile a list of what parts are actually on what engine.
I remember reading somewhere that Silverton #1 got the 286 boiler when it went to Silverton and that the SRR boiler was on 286 when it went to the NCNG RR. Some sources then call NCNG 8 the former D&RG 42 while other sources call it the former 286. SRR #1 is almost always shown to be the former D&RG #42, so that makes it interesting as to which one was actually which.
The same goes for the 454 and the 464 as I have been told that the boiler on the 464 was actually from the 454 at one point.
It does seem that the frame numbers determine the locomotive but since the builders plate went with the boiler, other people don't see it that way.