Robin, it depends who put the number on. Your number could be one from a club, or if made by a professional boiler maker could have his or her mark on it.
Most Model engineering clubs who carry out boiler tests call for a number to be added for identification, if the boiler is not already numbered. The club or owner would stamp the boiler, and this could indicate the club that tests the boiler. A club would be happy to use a boiler makers number for I/D if there were one. A boiler makers number could have a makers code as well. Why not share the number with the community who may well be able to answer your question?
Chris Gunn