In most pics of the older models with the small grille, i notice that the vent flap on the passenger side is usually open but the one on the drivers side is closed. What was the reason for having just the one open ?
Heater vent for air flow : 
As Lance said the vent was for the heater and if i remember right the one on the drivers side was just a vent to the outside which could be opened by a lever,just kicked it open.
Hi All
The passenger side one had a small bracket there so you couldn’t shut it.
Cheers Rich
It was the same on the 290’s too.
Hey, The first models till about '70 could the flap be opened and closed by a cable button on the radio case.
In the cabs live it gave about 10 different lay outs of heating en ventilations systems, but no one was as good as Scania’s heating system. from the coming of the f10/12 series in '77/'78, Volvo took over ans was the better for me as Scania’s system from then on, even better as the 2 and 3 series too.
Eric,