The hole with two, one right behind the other is for the defrost cable, if I remember right there is a stud and dtent for the bracket.

Fig 101 shows it right next to the defrost cable.
http://chevy.oldcarmanualproject.com/accessories/1951am/51aim38.htm
But I guess that would all depend on the type of heater that was installed

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1950 chevy 3100