Chris, I've been looking at my 19 set stuff and can't quite figure it out. I can see how the short strap will hold down the power supply but the holes on the set are made for the strap to pull the other way. My variometer looks like it just attaches to the power supply box.
Do you have any pix of the mounting board? It must be a few inches wider than the set by the looks of things It looks like it will just hook into the one end of the mounts and the spring loaded clips will hold on the other end. It'd be nice to get some measurements for the board.
There's a NOS 19 set on ebay now.
19 set
The first pic I've posted is my stuff and the others are the ebay stuff.