Few boilers were exchanged among frames in the US. It was a common practice in the UK. This was recently covered in another thread.

Part swapping was very common among any line that had even a few copies in the same class. Central of Georgia affiliated shortlines shared a few identical ten wheelers through the end of steam, one remains in the Savannah History Museum, and has parts from 3 or 4 others of its mates on it. Appliances such as air compressors, dynamos, injectors, whistles, bells, etc were commonly exchanges and rebuilt and restocked rather than pulling the locomotive out of service while a part was overhauled.

dave

" Few boilers were exchanged among frames in the US." Never the less it did happen. Everyone's favorite PRR Pacific carries her original builders plates no. 3475 May 1918. However the boiler numbers are 3739,a boiler built in March 1923 for engine road number 3844.

My takeaway from the responses basically was that no matter how many parts I find that bear the numbers 304 or 604, that is irrelevant if I don't check the boiler number. And then if do check that number, some wag would make another comment disregarding that finding.

Occam's Razor applies here. The simplest answer is the one most likely to be true. A typographical error was made by Mr. King.

Oh and BTW, did I mention that I have copies of actual DMIR records that show that in their files it was 604 that was shipped back to Greenville?

No, you didn't. But you should have, to save yourself a lot of grief and angst.