Re: A possible explanation - another one

All of the T-12's, C-16 and C-19 classes were built with wrought iron lap seam boilers. These boilers were replaced in the 1912-19 time frame. The boilers that were not replaced on the C-18's as they were built with steel lap seam boilers in 1896. However in several cases of the C-18's, the pressure was reduced to 145.
I have seen documentation somewhere that the 42 got a new boiler about 1916. Perhaps it was part of the "deal" when the RGS traded the 3 Crystal River engines for the 40-42. It would appear that perhaps the C-17's were built with steel lap seam boilers that were deemed "good enough" in 1915, but as time went on it was decided to set them aside as motive power needs got fewer and there was more than enough modern or reboilered power available.