Down here the Loco number is the running gear not the Boiler.
excerpted from Wiki NZR A
B class page
here
The AB type boiler had a working pressure of 180psi, a standard across the type. These boilers were of standard construction, no matter which firm built them, and as such were interchangeable across any locomotives of the type. They were also similar to the boilers built for the WAB and WS class 4-6-4T tank locomotives in 1939 - in fact, the WAB boiler and AB boilers were the same, with those fitted to the WAB fitted with the necessary components to draw water from the locomotive's side tanks.
This type of boiler was also adapted for use on the Q and AA class 4-6-2 tender locomotives of 1901 and 1915 respectively when their original boilers wore out. In the case of the AA class, the boiler change was not necessitated by the condition of the boilers but due to the limitations of the original boilers. The replacement took place in the 1930s, and these locomotives gained new heights of reliability, before they were withdrawn in 1957 (at the same time as the Q class). All of these boilers were then put back into the AB class pool, and were reused on engines of that class.
In all, 6 separate classes used the standard AB boiler. When Q, AA and G class locomotives were scrapped in the 1950s, the boilers were overhauled to keep locomotives of A, AB, and WAB serviceable.