These are examples of "lunettes" (Field glass; Fr), Tom:

http://www.pongopix.com/aw/jb/chip65.jpg
http://www.pongopix.com/aw/jb/chip71.jpg
http://www.pongopix.com/aw/jb/chip70.jpg

This is a plaque where the actual artwork on the laminated plastic is carried through the lunette.
http://www.pongopix.com/aw/benedict/chip56.jpg

You can also go to Chequers Magazino at CHEQUERS.COM (Benedict; #3) and see more example of construction.

JB