Several years ago now, my Dad was building custom windows for an architectual contractor, they were also an outlet for Pella Windows
But to the point, Dad was asked to come-up with an opening Round Window, what he came up with was very similar to the one Dan found, above
The window frame was round, with one glass pane (just over half round) fixed in the upper position, the lower pane again (just over half round) would rotate to cover the upper pane leaving the lower half open.
There was no locking device used for these, seeing as they were for a dormer on the 2nd floor.
I have no pictures and Dad is gone, but look at Dan's photo and remember all work was accomplished on a router table.
Someone out there should be able to come up with a fairly simple design.
