I am quite certain the subase was only used on the 6-7 hp engines. Perhaps for casting-assembly ease, or to make them fit the buzz saw rigs.You don't see many bases because most of the engines had the feet cast as part of the crankcase, only a few had the separate base. If you look closely at the photos you will see that the crankcase has the mounting lugs just below crank shaft height and that is also why the base has the cut outs so that it will clear the lower part of the crankcase which drops inside the separate base.
It is not like your freelance base that fits to the bottom of the cast feet making the engine top heavy.
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