there is a spring on the top of the front standard.
to remove the board you hold the lens, and pull up,
the bottom of the board will come out of a groove it is set into.
the bottom of the board is "pointy" the other 3 sides are squared off.
Thanks. Basically, the same design as the one from an Anniversary graphic but somewhere in between the one from a 3x4 (3 1/4") and the one from a 4x5 (4"). Very inconvenient of them.
I've been making them out of 4" x XXX" hobby board from the hardware store. I just saw off about 1/4" or so around the inside edges on one side and drill a hole in the middle. I faced one with brass, the rest are all wood.