Are we sure it doesn't (or didn't) open up to a ninety degree angle?
I haven't seen a pic of it fully open. When it's closed, it looks like (at the top) two forty fives, which should allow for a ninety when fully open. Of course, I wouldn't be at all suprised if the curved rule is bowed from age that it would hang up near the end and not allow for the piece to open all the way.
When this was used, it should have been able to open far enough for all of the measurment lines on the curved ruler to be on the inside of the angle, and for the hinge to be fully closed.
ppgs, will this thing still open all the way, or is it open as far as it will go in the pics?