I thought these control valves were just ball valve style and not directional and it would not matter which side of the heater core you installed it on.
I looked at the one I got from amazon. Note the arrow is reversed from the picture mr48chev posted.
Looking inside a few things strike me. Not sure what they mean in terms of fluid flow.
The opening in the valve core is offset to one side
It appears the widest open setting is before the control lever is all the way to open.
When it is off you can't blow through it from either side
The outlet side of the valve core has a unique shape
inlet side is round
outlet side is notched. Maybe the notch allows greater control at low flow rates?
Nothing I see indicates that this does anything more than stop the flow at off