The D&RG/D&RGW like other roads would often separate the boiler from the frame when a locomotive would go into the shop. This particularly is the case when either heavy boiler and/or frame work would be required. If your fleet of locomotives is large enough you may have several locomotives of the same kind in the shop at the same time.
Then often the first boiler completed would be matched with the first frame, etc. that was ready to go out the door. The number often remained with the frame, wheels, cylinders (the engine) of the the locomotive. So the end result is that the boiler has been moved to another locomotive.
This particularly occurred when the Rio Grande was repairing and replacing the boilers and fireboxes of the class 60 locomotives. I am told that this project occurred after a boiler watchman's fire rake went the side sheet of a firebox. The C-16 that was sold to the Nevada County Narrow Gauge carried the boiler from another C-16.
In case you wonder, Baldwin used a completely different number as it's identification number. This number would be stamped onto various parts of the frame, valve gear, tender shell, etc. This was the "class number." It did keep a cross reference between the class number and the boiler builder's plate.
Brian Norden