What is hinged board over window sill?

I've been thinking about installing replacement windows or sash guides in a sunroom of our 90 year old house. The 13 windows in this room have a curious board that is hinged to the window stool and lays on top of the sill. Because of them the lower sash does not close below the stool, but is level with it.
Can someone tell me what these hinged boards are for? Here's a link to a photo: www.adherentlabs.com/window.jpg.
Thanks, Tom

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