Those drawings are marked Caterpillar Tractor Co. I had thought that the armour was designed, made and fitted in the UK, possibly by Jack Olding Ltd. I can't see a reason for Cat to give a separate code to armoured tractors and only add 'SP' to other variants like pipe cranes which are just as different to base machines. There were lots of standard 7M D7s used by the military so it isn't because they were military contract.
David
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