Until the Holden period, there were only two or three boiler types that fitted more than one locomotive class. Boilers generally tended to remain with the engines to which they were originally fitted, but exchanges of boilers between engines of the same class were by no means uncommon. Although reasonable records of engines repaired at Stratford Works from 1875 onwards have survived, until after 1900 locomotives also underwent major repairs - including boiler changing - at Norwich, for which there are no known records. The Stratford Works records that remain show that there were a number of instances in which boilers were 'shoe-horned' into locomotives of another class.
A Boiler Explosions Act became law at the time that James Holden became Locomotive Superintendent. One of its provisions was that all boilers had to be individually numbered, and records kept of the repairs and alterations to each one. To simplify matters, the GER decided that each boiler was to be given the same number as the engine to which it belonged, and that engine and boiler would remain together for life. If the engine were to be renumbered subsequently, the boiler number was altered to correspond. When a boiler was life-expired and replaced by a new one, the replacement would again carry the same number as the engine.
By the period shortly before the First World War, this policy was causing delays in returning overhauled engines to traffic. The number of standard boiler types in use had been reduced to about half-a-dozen, whilst the number of locomotives in stock had practically doubled. It became increasingly common for engines to be held at the works awaiting the completion of repairs to their boilers, whilst out-shopped boilers of the same type were waiting for work on their engines to be finished. Thus, from early in 1913 it was decided that locomotives would be out-shopped with whatever boiler was available of the correct type.
The existing boilers kept their numbers, and it was still the rule that boilers fitted to new engines would have the same number as the locomotive. However, boilers built as replacements for existing engines were numbered in a new series from 2000 upwards. By this means, it was ensured that no two boilers would carry the same number. If for example, loco 478 was fitted with a new boiler, the new one might be numbered 2338. The engine's original boiler, still numbered 478, could then be re-used on another engine.
Hitherto, the GER regarded as locomotive as having been 'rebuilt' when fitted with a new, replacement boiler, and this fact was recorded on the loco numberplate. This remained the custom, well into the LNER period. However, under the new system, whether a locomotive received a new boiler or a second-hand one was largely a matter of chance, and depended upon what was to hand. Thus, some engines were 'rebuilt' twice in as many years, whilst others might never be accorded the accolade.
The new system of allocating boiler numbers broke down during the First World War. From 1916 onwards most of the new locomotives constructed were turned out with the 'wrong' boiler; either one that was intended for another member of the class, or one numbered in the 'replacement' series. Three S69 4-6-0s even managed to be built with the boilers intended for D81 0-6-0s. It follows that boilers intended for new engines - and numbered as such - were used to 'rebuild' existing locomotives. It would appear that new boilers were only nominally allocated to specific engines and numbered accordingly, or were otherwise numbered in the replacement series. Whether or not they were fitted to the correct engines did not matter, provided that the numbers borne by the boilers were not duplicated.