The one I have on a Birmingham trade gun is pretty straight forward. The top lever has a small leg at the front which goes into a slot in the cross bolt. It's all hidden just off centre of the top of the breach. When the lever is moved to the right, to open the gun, the leg at the front of the opening lever pushes the cross bolt to the left and clear of the hole in the rib extension that sits in the slot between the fences at the top of the action so the barrels are free to open, some of the cross bolts stick out of the side of the fence when open, others are hidden inside. There are also different mechanisms for operating them, some fancy, some not so fancy. Whether or not they really contribute to the strength of the lock up is open to argument, many different styles have been advertised as the best and strongest yet over the years but it's know that many were over complicated or offered no real advantage .
I believe the trick when fitting them is to only allow the bolt to fit against one side of the hole in the extension but can't remember where I heard it.
I hope someone with the know how may add to that as I'm not sure what I've written will make sense now I've looked through it
edit, sorry Redoak, didn't read your post before I scribbled mine down.
I've altered my post too after noticing a mistake.
