This is what I have on the shelf. I am no Mamod expert, all I know is what I have read over the years... and from what I have read, that would make these first two pre war Minors as the engine frames are bare brass with silver crank disks and the third post war.
From left to right.
First: has silver painted crank disk, and a steel boiler band with remnants of black paint (9x5 holes)
Second: crank disk is bare metal and the firebox/boiler band have had the paint sanded away by a previous owner (9x5 holes)
Third: I had thought this was maybe pre war, but upon close examination I can find very trace remnants of green paint on the engine frame and a few speckles of gold paint on the "Mamod Minor" stamped crank disk, making it a post-war with a 10X6 hole base. This also has the copper-plated steel boiler band.
Also note that the two pre-wars have a lighter orange-red than my post-war example. I can see a few spots of new red paint on the later one, but it is hard to tell if it's had a repaint at some point... if it has, it was many, many years ago...
The twin was sold with both big and small baseplates.