Re: Boiler repairs question

I interpret Roger's question as regarding the outside appearance of the dome. This is actually the jacket, whether it is sheet metal or cast iron, it is still only a jacket over the dome itself, which is rolled steel plate like the boiler. Different jackets may have been used at different times due to rust through, damage due to rollover, etc, but it would have been very rare to replace the actual dome itself without replacing the entire boiler. And yes, there is insulation between the dome and the jacket; it would have been the same insulation as used on the rest of the boiler. So any difference seen in photos taken at different times was probably either due to different dome jackets, or perhaps a complete boiler replacement, in which case the dome on the new boiler may have been in a slightly different location.
Hope that helps.
Chris