It would depend on who made the boiler, but it was probably obvious and legible to them… until someone went and built a road-locomotive around it.
First and easiest places to look are around the cylinder-mounting, and the boiler crown in the depths below the crankshaft. Some makers fit an ID plate in the latter position.
The numbers might be stamped along the topmost back or front edge of the outer firebox, though there it could be obscured by the plate-work.
Other likely positions are around the top of the backhead itself, possibly hidden by the superstructure and fittings. Along the bottom edge of the backhead deep below the man-stand – so possibly obscured by the water-tank. Or the bottom edge of the throat-plate – possibly obscured by the steering-gear or ashpan assemblies.
Just possibly, on the front or back inner-firebox walls if these protrude below the foundation-ring: not good practice as they would be hidden by the ashpan at least, on the complete engine.
The stampings would not, or ought not, be on single-thickness plate regions but on the outside of flanged or welded seams, including the foundation-ring. So I'd use those as a guide if the first two places don't yield anything.
If nowhere there, we start thinking of awkward places like the area of the smoke-box, and hope whoever numbered the boiler did think not to place it where the horn-plates would cover it.
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If these don't bear fruit…
Was the boiler commercially made? If so and the company still exists, try asking them. They might use their own standard locations.
Another source you could ask, if you are not already a member anyway, is the Model Road Steam Vehicle Society. Their members between them own such a range of engines it's quite possible someone will know more definitely where it is on yours, at least for a professionally-built boiler. (Tell 'em a member suggested it!) Though still be aware that different boiler-makers have their own sites for the I.D.
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I am not sure if it's asked for on any of the increasingly convoluted MELG paperwork, but the location of the number ought be noted on the individual boiler's documents somewhere. A good place would be on the Written Scheme of Examination, as that lists the boiler's test details. .