I'm sure there are a couple more that I noticed just the general descriptions are probably from some librarian/photo scanning person. that doesn't have a clue or was trying to interpret the writing on the back of the photo's.
They used lower super heat temperatures, then a lot of equipment did. Having not had to deal with super heat for the last 25 years of my boiler /engineer operating career. I can say that I would have to dig through the books to get the numbers of what and the temp points. It wasn't a fun subject the first time around when I wrote for my license. 34 years ago.
Knowing how small museums work, they used the same description when they didn't have one or a clue
Ok ya got me! I think this will give the basic answer. In the US back in the day the max operating superheat temps were 600'F other than for some composite tandem type engines at the Ford plant here in Detroit were the steam temperature was 700'F
Well here is a brief idea:
Because the steam is dry, it behaves more like a gas, no condensation as the engine absorbs heat from the steam. The economy increase is based on the fact that if the superheat is high enough not only to supply the heat absorbed by the cylinder walls but also the heat equivalent of the work done during the expansion, then the steam will be dry and saturated at release.
This is the condition of maximum efficiency in a single cylinder.
A superheat of 250'F to 350'F at admission is necessary to in sure the steam is dry at release in the average single cylinder engine.
Another advantage for piston engine work, A moderate amount of superheat produce a large increase volume, the pressure remaining constant, and diminishes the weight of the steam per stroke for a given amount of work.
And it has a lower thermal conductivity, less heat is absorbed by the cylinder walls.
Steam Power Plant Engineering, By G F Gebhartdt, 1922 edition
