There is no visable difference in size between front and back cilinders.
Sorry, Palmerston but there certainly is - on LGB and the original.
ON the LGB and Regner´s livesteam model the backcilinders protude outside the loco´s body.
The original being 750mm gauge, they wouldn't fit completely under the tanks - rubber ruler
On the real loco they are within the width of the water tanks.
So, LGB and Regner fooled us. No compound!
?
The back set of wheels arent even in a truck, they are part of the frame and placed inside to accomodate space for the firebox.
I think you are confusing the Mayer with a Mallet - the rear truck is a truck and can swivel on the Mayer.
Only if both drivetrains are in sync one can hear four beats. This isnt likely.
Check
accucraft.de for a prototypical and (livesteam) model.