To answer my own question:
It doesn't look like the two parts are supposed to be joined by a bolt. However, after mounting the heater unit and the flap control panel, the two holes (white arrows) lined up with the lower of the two holes in the heater unit (shown by yellow arrow [the upper hole is not visible on photo]). So why not bolt them together and make the panel more sturdy?
To bolt the two parts together, I had to make a 19 mm spacer for the right side and a 9 mm spacing for the left side.
Heater panel.jpg
The spacers look like this.
Spacer.jpeg
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