Bill,
Depending on what make of combi you buy, it is different. Newer Isabellas look like combis but are really a normal porch type roof. Older ones you did have to measure and then cut the bead. I wouldn't recommend opening the awning rail up for two reasons; it looks a mess and will never straighten properly again and, if you don't cut the bead, the downward pressure will cause the gap to widen and the roof will pull out of the rail - especially if it is windy. It takes a bit of courage to cut the bead, but it does look and fit better and doesn't damage the look of the 'van.
As abve, some versions look like you have to cut the bead / open the rail but they are really a conventional porch type roof in disguise.
Neal