D Series
Just as a point of interest the "D" in Caterpillar stands for Diesel or so I'm told.... The 1931 "Diesel 60" motor built by Cat became the staple heavy machine engine used in the USA at least.
The number designated size as already mentioned..
The 1942 RD-1820 radial diesel engine was used to power the M-4 tank with the "RD" prefix standing for Radial Diesel..
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Mallee Wombat
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