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There are a few differences between the Mallard and Scotsman.
The cylinders on the Scotsman are attached entirely to the chassis.
The Mallard has a cover which fits on with the body shell.
The valve gear on the Scotsman looks very similar, but they have
subtle differences. Although the Mallard gear can be made to
fit the Scotsman, it doesn't look right.
Here is the Mallard valve gear for comparison. Look at the line
of the top rail.
Note also the cylinder shroud.
Other differences are even more subtle. Gone is the little
spring to connect the earth to the tender - instead, the motor earth
is soldered to a brass earthing plate, and there is no bulb
wired in to the Scotsman. Apart from that, the two models are
very similar, and parts are interchangeable.
Apart from the wheels on the Gresley (red) and the 4472 Scotsman
(green) - but these are just cosmetic differences.