My WAGR sources indicate that the "W"s in use now were the best remainening at
the end of steam (with one or two exceptions eg 907).
most were overhauled at the end of steam ac the Vietnam War. The WAGR didn't
want to get caught with its pants down as it had for WWII.(in terms of power
availability).
Further, those sent for preservation, whether PRRPS HVTR ARHSWA SPRPS or the
Great Southern, were a combination of the best bits and pieces (eg
tender/engine) that were available at the time.
As they were all destined to become your new
Hyundai/Daiwoo/Nissan/Suzuki/Toyota/Honda/Daihatsu etc etc, most came with a
few extra bits in the firebox and/or coal space.
There were spare boilers sent out from BP that allowed rotation as repairs were
made, thus (and this would be the case for most locomotives) the boiler numbers
don't necessarily match the cab/frame numbers, especially given the problems
with boilers that were experienced in the early days of the class.