wouldnt a chain under the tracks affect how the tracks drive? or cause damage to the drive wheel? and it would be a bitch trying to lift the tracks to run a chain underneath.
i would not be surprised if they used a 'shoe' or 'clamp' similar to the one for torpedoes or spuds
in gwyn's photo, 2 small chains (or 1 small chain) could be connected to the sides of a shoe, and then the chain to the undititching beam is connected to those with a shackle. it might be my imagination, but it looks as though there may be something in the middle of the track plate that the chain is attached to; maybe the central bar of a shoe?
the maintenance manual for the mark four is available, though, maybe it has a set of instructions for attaching the unditching gear.
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