Someone on a factory line somewhere punched a hole in the wrong spot on the frame during manufacturing. He discussed the issue with his shop foreman and the foreman agreed "screw it let's run with it" making all the frames the same with the mystery hole.
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Daniel G.
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(May 1970) 147 w/an IH spring assist, 48" deck, 42" blade, 1969 73, #2 trailer, 10" Brinly plow and (on loan) Dad's #2 tiller.