I'm not to sure during the 1960s when 3830 was aquired by the Museum of Applied Arts and Sciences for preservation if it was intended to ever be a trafficable exhibit, hence no spare boiler was obtained.
But you really cant excuse the poor management of that era by the NSW Rail Transport Museum in not aquiring a spare 38 class boiler or two when they may have been available as freebies from the Dept. or for purchase when 38 class were disposed of as scrap for $5000
No such poor management. Items for retention at that time were decided upon by the Department, even if recommended by RTM. Witness a lack of D-58 Class nowadays or a similar lack of NCR Sets and Enfield Locomotive Depots.
Remember that there were 35 boilers for the 30 locos. Each boiler was reaching the end of its life as they were condemned and then scrapped. The last one to be scrapped was Boiler 3815 in Locomotive 3818 in early 1971. Since each boiler was in poor condition as it was withdrawn, the Department did not keep any spares. You have to remember that RTM was casting around for a new 38 Class boiler in the early to mid 1970's, being told by one UK firm, misunderstanding what was being asked for, that it would be cheaper to get them to recondition a Castle from Barry.
The four boilers left, 3801a (3830), 3803a (3813), 3819 (ex-3801, now 'spare') and 3825a (3820) were among the best ones at the time. In some ways, we are lucky that we have three of the five spare boilers because they are the youngest of the lot.
3820 is only still with us due to excellent management and foresight during that era by the NSW Rail Transport Museum. Without a sound, reasoned argument in favour of retaining it for traffic purposes, that was mounted by the NSW Rail Transport Museum, it would have gone to Sims very quickly after 29/12/1970. Indeed, it could have gone there coupled to 3818. In some ways, you could call it a defacto spare boiler, fitted to a full set of spare components that rather happily make up a complete locomotive.
The only reason that we have spare 36 Class boilers is that they were relatively new, having between 5 to 12 years of use. Hence many were kept for stationary boiler use, replacing a motley collection of older boilers in the process.